Add contributing md (#279)

* Add contributing setup file

* Add sandbox method

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# Contributing
+
+
+## Setup
+
+### Option 1 - Manual
+
+1. Fork and clone the repo
+1. Run `npm install` to install dependencies
+1. Create a branch for your PR with `git checkout -b your-branch-name`
+
+> To keep `master` branch pointing to remote repository and make
+> pull requests from branches on your fork. To do this, run:
+>
+> ```sh
+> git remote add upstream https://github.com/excalidraw/excalidraw.git
+> git fetch upstream
+> git branch --set-upstream-to=upstream/master master
+> ```
+>
+
+### Option 2 - Codesandbox
+
+1. Go to https://codesandbox.io/s/github/excalidraw/excalidraw
+1. Connect your Github account
+1. Go to Git tab on left side
+1. Tap on `Fork Sandbox`
+1. Write your code
+1. Commit and PR automatically