add CONTRIBUTING

CONTRIBUTING.md

@@ -0,0 +1,30 @@
+# Contributing
+
+## Report a bug or propose a new feature
+
+Open a new issue from [issues](https://github.com/ueokande/vim-vixen/issues).
+**Ensure the issue was not already reported** by searching on GitHub under Issues.
+The issue should include title and clear description.
+
+Pull request is also welcome to send a patch from [Pull Requests](https://github.com/ueokande/vim-vixen/pulls).
+Ensure the pull request includes description, and passes tests in CI.
+
+## Start a development
+
+Clone sources into local
+
+    git clone https://github.com/ueokande/vim-vixen
+
+Install dependencies:
+
+    npm install
+
+Start webpack:
+
+    npm run start
+
+Then open `about:debugging` in Firefox, and choose directory from "Load Temporary Add-on".
+To run tests and lint:
+
+    npm run test
+    npm run lint