Add CONTRIBUTING guidelines

CONTRIBUTING.md

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+Contributing
+============
+
+All contributions are welcome to this project.
+
+Contributor License Agreement
+-----------------------------
+
+Before a contribution can be merged into this project, please fill out the
+Contributor License Agreement (CLA) located at:
+
+http://opensource.box.com/cla
+
+To learn more about CLAs and why they are important to open source projects,
+please see the [Wikipedia entry][1].
+
+How to contribute
+-----------------
+
+* **File an issue** - if you found a bug, want to request an enhancement, or
+  want to implement something (bug fix or feature).
+* **Send a pull request** - if you want to contribute code. Please be sure to
+  file an issue first.
+
+Pull request best practices
+---------------------------
+
+Following these steps will help ensure that your pull request gets reviewed and
+accepted as quickly as possible.
+
+### Step 1: File an issue
+
+Before writing any code, please file an issue stating the problem you want to
+solve or the feature you want to implement. This allows us to give you feedback
+before you spend any time writing code. There may be a known limitation that
+can't be addressed, or a bug that has already been fixed in a different way. The
+issue allows us to communicate and figure out if it's worth your time to write a
+bunch of code for the project.
+
+### Step 2: Fork this repository in GitHub
+
+This will create your own copy of our repository.
+
+### Step 3: Add the upstream source
+
+The upstream source is the project under the Box organization on GitHub. To add
+an upstream source for this project, type:
+
+```
+git remote add upstream [email protected]:box/box-java-sdk.git
+```
+
+This will come in useful later.
+
+### Step 4: Create a feature branch
+
+Create a branch with a descriptive name, such as `add-search`.
+
+### Step 5: Push your feature branch to your fork
+
+As you develop code, continue to push code to your remote feature branch. Please
+make sure to include the issue number you're addressing in the body of your
+commit message and adhere to [standard git commit message guidelines][2]. For
+example:
+
+```
+Add search
+
+Introduced a new BoxSearch class that uses the /search API endpoint.
+
+Closes #123.
+```
+
+This helps us out by allowing us to track which issue your commit relates to.
+
+Keep a separate feature branch for each issue you want to address.
+
+### Step 6: Rebase
+
+Before sending a pull request, rebase against upstream, such as:
+
+```
+git fetch upstream
+git rebase upstream/master
+```
+
+This will add your changes on top of what's already in upstream, minimizing
+merge issues.
+
+### Step 7: Build and test your changes
+
+Make sure that the code builds and passes all unit tests by running:
+
+```bash
+$ gradle clean build
+```
+
+### Step 8: Send the pull request
+
+Send the pull request from your feature branch to us. Be sure to include a
+description in your commit message (not the pull request description) that lets
+us know what work you did.
+
+Keep in mind that we like to see one issue addressed per pull request, as this
+helps keep our git history clean and we can more easily track down issues.
+
+[1]: http://en.wikipedia.org/wiki/Contributor_License_Agreement
+[2]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html