docs: Add README file that describes project usage & background

README.md

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+buildGo.nix
+===========
+
+This is an alternative [Nix][] build system for [Go][]. It supports building Go
+libraries and programs, and even automatically generating Protobuf & gRPC
+libraries.
+
+*Note:* This will probably end up being folded into [Nixery][].
+
+## Background
+
+Most language-specific Nix tooling outsources the build to existing
+language-specific build tooling, which essentially means that Nix ends up being
+a wrapper around all sorts of external build systems.
+
+However, systems like [Bazel][] take an alternative approach in which the
+compiler is invoked directly and the composition of programs and libraries stays
+within a single homogeneous build system.
+
+Users don't need to learn per-language build systems and especially for
+companies with large monorepo-setups ([like Google][]) this has huge
+productivity impact.
+
+This project is an attempt to prove that Nix can be used in a similar style to
+build software directly, rather than shelling out to other build systems.
+
+## Example
+
+Given a program layout like this:
+
+```
+.
+├── lib          <-- some library component
+│   ├── bar.go
+│   └── foo.go
+├── api.proto    <-- gRPC API definition
+├── main.go      <-- program implementation
+└── default.nix  <-- build instructions
+```
+
+The contents of `default.nix` could look like this:
+
+```nix
+{ buildGo }:
+
+let
+  api = buildGo.grpc {
+    name  = "someapi";
+    proto = ./api.proto;
+  };
+
+  lib = buildGo.package {
+    name = "somelib";
+    srcs = [
+      ./lib/bar.go
+      ./lib/foo.go
+    ];
+  };
+in buildGo.program {
+  name = "my-program";
+  deps = [ api lib ];
+
+  srcs = [
+    ./main.go
+  ];
+}
+```
+
+(If you don't know how to read Nix, check out [nix-1p][])
+
+## Usage
+
+`buildGo` exposes five different functions:
+
+* `buildGo.program`: Build a Go binary out of the specified source files.
+
+  | parameter | type                    | use                                            | required? |
+  |-----------|-------------------------|------------------------------------------------|-----------|
+  | `name`    | `string`                | Name of the program (and resulting executable) | yes       |
+  | `srcs`    | `list<path>`            | List of paths to source files                  | yes       |
+  | `deps`    | `list<drv>`             | List of dependencies (i.e. other Go libraries) | no        |
+  | `x_defs`  | `attrs<string, string>` | Attribute set of linker vars (i.e. `-X`-flags) | no        |
+
+* `buildGo.package`: Build a Go library out of the specified source files.
+
+  | parameter | type         | use                                            | required? |
+  |-----------|--------------|------------------------------------------------|-----------|
+  | `name`    | `string`     | Name of the library (and resulting executable) | yes       |
+  | `srcs`    | `list<path>` | List of paths to source files                  | yes       |
+  | `deps`    | `list<drv>`  | List of dependencies (i.e. other Go libraries) | no        |
+  | `path`    | `string`     | Go import path for the resulting library       | no        |
+
+* `buildGo.external`: Build a Go library or program using standard `go` tooling.
+
+  This exists for compatibility with complex external dependencies. In theory it
+  is possible to write `buildGo.package` specifications for each subpackage of
+  an external dependency, but it is often cumbersome to do so.
+
+  | parameter | type           | use                                            | required? |
+  |-----------|----------------|------------------------------------------------|-----------|
+  | `path`    | `string`       | Go import path for the resulting library       | yes       |
+  | `src`     | `path`         | Path to the source **directory**               | yes       |
+  | `deps`    | `list<drv>`    | List of dependencies (i.e. other Go libraries) | no        |
+  | `srcOnly` | `bool`         | Only copy sources, do not perform a build.     | no        |
+  | `targets` | `list<string>` | Sub-packages to build (defaults to all)        | no        |
+
+  For some examples of how `buildGo.external` is used, check out
+  [`proto.nix`](./proto.nix).
+
+* `buildGo.proto`: Build a Go library out of the specified Protobuf definition.
+
+  | parameter   | type        | use                                              | required? |
+  |-------------|-------------|--------------------------------------------------|-----------|
+  | `name`      | `string`    | Name for the resulting library                   | yes       |
+  | `proto`     | `path`      | Path to the Protobuf definition file             | yes       |
+  | `path`      | `string`    | Import path for the resulting Go library         | no        |
+  | `extraDeps` | `list<drv>` | Additional Go dependencies to add to the library | no        |
+
+* `buildGo.grpc`: Build a Go library out of the specified gRPC definition.
+
+  The parameters are identical to `buildGo.proto`.
+
+## Current status
+
+This project is work-in-progress. Crucially it is lacking the following features:
+
+* feature flag parity with Bazel's Go rules
+* documentation building
+* test execution
+
+There are still some open questions around how to structure some of those
+features in Nix.
+
+[Nix]: https://nixos.org/nix/
+[Go]: https://golang.org/
+[Nixery]: https://github.com/google/nixery
+[Bazel]: https://bazel.build/
+[like Google]: https://ai.google/research/pubs/pub45424
+[nix-1p]: https://github.com/tazjin/nix-1p