Add basic build instructions (tailwindlabs#830)

* Add basic build instructions

* Create `CONTRIBUTING.md`

---------

Co-authored-by: Brad Cornes <hello@bradley.dev>

Author: flostellbrink

.github/CONTRIBUTING.md

@@ -0,0 +1,19 @@
+# Contributing
+
+Thanks for your interest in contributing to Tailwind CSS IntelliSense! Please take a moment to review this document **before submitting a pull request**.
+
+## Pull requests
+
+**Please ask first before starting work on any significant new features.**
+
+It's never a fun experience to have your pull request declined after investing a lot of time and effort into a new feature. To avoid this from happening, we request that contributors create [a feature request](https://github.com/tailwindlabs/tailwindcss/discussions/new?category=ideas) to first discuss any significant new ideas.
+
+## Building the Extension
+
+You can build the VSIX package by running these commands in the project root.
+
+```bash
+npm install
+npm run bootstrap
+npx lerna run package --scope=vscode-tailwindcss
+```