Create CONTRIBUTING.md

New CONTRIBUTING.md with release process

CONTRIBUTING.md

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+## Release process
+To perform the release you must have write access to this project and be added 
+as collaborator in [NPM for gulp-bles](https://www.npmjs.com/package/gulp-bless).
+
+1. Pull `master` branch and ensure you have latest changes.
+1. Run `npm test` to make sure all the tests are passing.
+    * If there are test failures fix them, commit, push, and start from step 1.
+1. Increase the version in package.json. See [semver.org](http://semver.org/) to figure out which part of the version string to increase.
+1. Login to NPM by using the command `npm login`, you can use `npm whoami` to check your login status.
+1. Run `npm publish`
+1. Commit and push the your changes in `package.json`
+1. Go to the GitHub project and create a release (see the releases tab). 
+    * Use the release version as title, e.g. "3.2.1". 
+    * Make sure the branch is master, add a quick description of the changes in the release (for example: "Fix for blah blah <link to issue>").
+    * Pre-release checkbox shouldn't be checked.