Create CONTRIBUTING.md

Author: glibas

CONTRIBUTING.md

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+# Contributing to Selenium Shutterbug
+
+The Selenium Shutterbug project welcomes contributions from everyone. 
+
+### Step 1: Fork
+
+Fork the project [on Github](https://github.com/assertthat/selenium-shutterbug)
+and check out your copy locally.
+
+```text
+% git https://github.com/assertthat/selenium-shutterbug.git
+% cd selenium-shutterbug
+% git remote add upstream git://github.com/assertthat/selenium-shutterbug.git
+```
+### Step 2: Branch
+
+Create a feature branch and start improving:
+
+```text
+% git checkout -b my-feature-branch
+```
+
+HEAD-based development is prefered, which means all changes are applied
+directly on top of master.
+
+### Step 3: Commit
+
+If you hve not configured Git already, its right time to do it, by specifying your name and email:
+
+```text
+% git config --global user.name 'Your Name'
+% git config --global user.email 'name@example.com'
+```
+
+Please read this before writing commit message:
+
+http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+
+### Step 4: Rebase
+
+Use `git rebase` (not `git merge`) to sync your work from time to time.
+
+```text
+% git fetch upstream
+% git rebase upstream/master
+```
+
+### Step 7: Push
+
+```text
+% git push origin my-feature-branch
+```
+
+Go to https://github.com/yourusername/selenium-shutterbug and press the _Pull
+Request_ and fill out the form. 
+
+You PR will be reviewed ASAP and integrated to master.