docs: add contributing section and commit hooks

Author: mhluongo

.githooks/pre-commit

@@ -0,0 +1,7 @@
+#!/bin/sh
+set -e
+
+cargo fmt --all
+git add -u '*.rs'
+npx prettier --write '**/*.md'
+git add -u '*.md'

.prettierrc

@@ -0,0 +1,4 @@
+{
+  "proseWrap": "always",
+  "printWidth": 80
+}

README.md

@@ -89,3 +89,20 @@ age-plugin-argon2 --list -i key.txt
 
 Licensed under [MIT](LICENSE-MIT) or [Apache 2.0](LICENSE-APACHE), at your
 option.
+
+## Contributing
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
+
+### Pre-commit hooks
+
+Consider installing the commit hooks before your first commit:
+
+```sh
+git config core.hooksPath .githooks
+```
+
+The pre-commit hook runs `cargo fmt` and `prettier` on Markdown files. You'll
+need [Node.js](https://nodejs.org) available for the latter.