docs(contributing): add local setup and publishing docs

Author: imhoffd

CONTRIBUTING.md

@@ -0,0 +1,41 @@
+# Contributing
+
+:mega: **Support/Questions?**: Please see our [Support
+Page](https://ionicframework.com/support) for general support questions. The
+issues on GitHub should be reserved for bug reports and feature requests.
+
+### Local Setup
+
+1. Fork the repo & clone it locally.
+1. `npm install`
+1. `npm run build` to build source files
+
+#### Warning: `npm link` may not work
+
+Using symlinks for Angular tooling may lead to issues. To test changes in a real project, copy the library to `node_modules/@ionic/angular-toolkit`. For example, using `rsync`:
+
+```
+rsync -avuP --exclude .git --exclude node_modules /path/to/git/angular-toolkit node_modules/@ionic
+```
+
+### Publishing
+
+CI automatically publishes the next version semantically from analyzing commits in `stable`. To maintain a shared history between `master` and `stable`, the branches must be rebased with each other locally.
+
+* When it's time to cut a release from `master`:
+
+    ```
+    git checkout stable
+    git rebase master
+    git push origin stable
+    ```
+
+* Await successful publish in CI. Ionitron will push the updated versions and tags to `stable`.
+* Sync `master` with `stable`.
+
+  ```
+  git pull origin stable
+  git checkout master
+  git rebase stable
+  git push origin master
+  ```