docs(tazjin/niri-reap): add a simple README

Change-Id: I53c760115bda3df2f964ac359f4c9e127c9890ad
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12565
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

users/tazjin/niri-reap/README.md

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+niri-reap
+=========
+
+Tiny, MIT-licensed companion program for [niri](https://github.com/YaLTeR/niri).
+
+I don't use workspaces in my workflow, but when disconnecting an external
+screen, the workspaces that it was displaying are moved to the remaining screen.
+
+This program "reaps" all windows on workspaces except the currently active one,
+and moves them all to the current workspace.
+
+## Usage
+
+If you have the full TVL monorepo, just `mg run //users/tazjin/niri-reap`. There
+is no configuration, and there are no flags.
+
+If you don't have the TVL monorepo and just want `niri-reap`, do this:
+
+1. Get the code: `git clone https://code.tvl.fyi/depot.git:/users/tazjin/niri-reap.git`
+2. Run the code: `cargo run`