docs: Add README with a bit of explanation

README.md

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+defer in Rust
+=============
+
+After a Hacker News discussion about implementing Go's `defer` keyword in C++,
+I stumbled upon [this comment](https://news.ycombinator.com/item?id=15523589)
+and more specifically this response to it by "Occivink":
+
+> There's plenty of one-time cases where you don't want to declare an entire
+> class but still enjoy scope-based functions.
+
+Specificall the "don't want to declare an entire class" suggests that languages
+like C++ have high friction for explaining your desired invariant (cleanup is
+run when `$thing` is destroyed) to the compiler.
+
+It seems like most languages either hand-wave this away (*cough* Java *cough*)
+or use what seems like a workaround (`defer`).
+
+Rust has the so-called `Drop` trait, which is a typeclass that contains a single
+method with no return value that is run when a variable is dropped (i.e. goes out
+of scope).
+
+This works fine for most general cases - i.e. closing file handlers - but can
+get complicated if other use-cases of `defer` are considered:
+
+* returning an error-value by mutating a reference in the enclosing scope (oh boy)
+* deferring a decision about when/whether to run cleanup to the caller
+
+While thinking about how to do this with the `Drop` trait I realised that `defer`
+can actually be trivially implemented in Rust, using `Drop`.
+
+A simple implementation of `defer` can be seen in [defer.rs](examples/defer.rs),
+an implementation using shared mutable state for error returns is in the file
+[defer-with-error.rs](examples/defer-with-error.rs) and an implementation that
+allows cleanup to be *cancelled* (don't _actually_ do this, it leaks a pointer)
+is in [undefer.rs](examples/undefer.rs).
+
+Whether any of this is actually useful is not up to me to decide. I haven't
+actually had a real-life need for this.
+
+You can run the examples with `cargo run --example defer`, etc.
+
+## Notes
+
+* `Drop` is not guaranteed to run in case of panics or program aborts, if you
+  need support for that check out [scopeguard](https://github.com/bluss/scopeguard)
+
+
+## Further reading:
+
+* [The Pain Of Real Linear Types in Rust](https://gankro.github.io/blah/linear-rust/)
+* [Go's defer](https://tour.golang.org/flowcontrol/12)
+* [Rust's Drop](https://doc.rust-lang.org/std/ops/trait.Drop.html)