docs(contributing): Add contribution notes

CONTRIBUTING.md

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+Hi collaborator!
+
+If you have a fix or a new feature, please start by checking in the
+[issues](https://github.com/algolia/algoliasearch-jekyll/issues) if it is
+already referenced. If not, feel free to open one.
+
+We use [pull requests](https://github.com/algolia/algoliasearch-jekyll/pulls)
+for collaboration. The workflow is as follow:
+
+- Create a local branch, starting from `develop`
+- Submit the PR on `develop`
+- Wait for review
+- Do the changes requested (if any)
+- We may ask you to rebase the branch to latest develop if it gets out of sync
+  with `develop`
+- Receive the thanks of the Algolia team :)
+
+# Development workflow
+
+After the necessary `bundle install`, you can simply launch `guard` to start the
+test suite in watch mode (perfect for TDD).
+
+If you want to test the plugin on an existing Jekyll website while developping,
+I suggest updating the website `Gemfile` to point to the correct local directory
+
+```ruby
+gem "algoliasearch-jekyll", :path => "/path/to/local/gem/folder"
+```
+
+If you plan on submitting a PR, I suggest you install the git hooks. This will
+run pre-commit and pre-push checks. Those checks will also be run by TravisCI,
+but running them locally gives faster feedback.
+
+# Tagging and releasing
+
+This part is for main contributors:
+
+```
+
+# Bump the version
+vim ./lib/version.rb
+git add ./lib/version.rb
+git commit -m 'Version bump to X.Y.Z'
+
+# Install the gem locally
+rake install
+
+# Release to Rubygems
+rake release
+```
+
+
+