Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contribution guidelines
+
+Strawberry is an free and open-source project, it is possible and encouraged to participate in the development.
+You can also participate by answering questions, reporting bugs or helping with documentation.
+
+
+## Submitting a pull request
+
+You should start by creating a fork of the Strawberry repository using the GitHub
+fork button, after that you can clone the repository from your fork.
+Replace "username" with your own.
+
+
+### Clone the repository
+
+    git clone git@github.com:username/strawberry.git
+    cd strawberry
+
+
+### Setup the remote
+
+    git remote add upstream git@github.com:strawberrymusicplayer/strawberry.git
+
+
+### Create a new branch
+
+This creates a new branch from the master branch that you use for specific
+changes.
+
+    git checkout -b your-branch
+
+
+### Stage changes
+
+Once you've finished working on a specific change, stage the changes for
+a specific commit.
+
+Always keep your commits relevant to the pull request, and each commit as
+small as possible.
+
+    git add -p
+
+
+### Commit changes
+
+    git commit
+
+
+### Commit messages
+
+The first line should start with "Class:", which referer to the class
+that is changed. Don't use a trailing period after the first line.
+If this change affects more than one class, omit the class and write a
+more general message.
+You only need to include a main description (body) for larger changes
+where the one line is not enough to describe everything.
+The main description starts after two newlines, it is normal prose and
+should use normal punctuation and capital letters where appropriate.
+
+An example of the expected format for git commit messages is as follows:
+
+```
+class: Short explanation of the commit
+
+Longer explanation explaining exactly what's changed, why it's changed,
+and what bugs were fixed.
+
+Fixes #1234
+```
+
+
+### Push the changes to GitHub
+
+Once you've finished working on the changes, push the branch
+to the Git repository and open a new pull request.
+
+
+    git push origin your-branch
+
+
+### Update your fork's master branch
+
+    git checkout master
+    git pull --rebase origin master
+    git fetch upstream
+    git merge upstream/master
+    git push origin master
+
+
+### Update your branch
+
+    git checkout your-branch
+    git fetch upstream
+    git rebase upstream/master
+    git push origin your-branch --force-with-lease
+
+
+### Rebase your branch
+
+If you need fix any issues with your commits, you need to rebase your
+branch to squash any commits, or to change the commit message.
+
+    git checkout your-branch
+    git log
+    git rebase -i commit_sha~
+    git push origin your-branch --force-with-lease
+
+
+### Delete your fork
+
+If you do not plan to work more on Strawberry, please delete your fork from GitHub
+once the pull requests are merged.