chore(readme): add install and usage sections

README.md

@@ -10,6 +10,24 @@ Supports:
 - multithreading
 - config file for saving common options
 
+## Install
+
+Download the [latest release][] or build from source using `cargo`.
+
+Make sure `git-vain` is in your `$PATH` or running `git vain` won't work.
+
+To build from source, run `cargo install --path .` or `cargo build --release`
+in your local checkout.
+
+[latest release]: https://git.anna.lgbt/anna/git-vain/releases/latest
+
+## Usage
+
+Replace the hash of the commit at `HEAD` with one that begins with `c0ffee`:
+`git vain c0ffee`
+
+Try using `--help` for more information.
+
 ## Performance
 
 Performance depends on hardware, method, and number of threads. The tests below