Add CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing
+
+You can contribute to Aseprite in several ways. One of the easiest
+ways to contribute is writting articles, blog posts, recording video
+tutorials, [creating art](http://aseprite.deviantart.com/), or showing
+your love to Aseprite e.g. naming Aseprite in your website and linking
+it to http://www.aseprite.org/, following
+[@aseprite](https://twitter.com/aseprite) twitter account, or
+[giving a donation](http://www.aseprite.org/donate/).
+
+Other ways to contribute require direct contact with us. For example:
+* Writting documentation.
+* Making art *for* Aseprite (logos, skins, mockups).
+* Sending patches for features or bug fixes.
+* Reviewing issues in the [issue tracker](http://code.google.com/p/aseprite/issues/list) and making comments.
+
+The following sections explain some tips about each of these points.
+
+## Documentation
+
+You can start seeing the
+[Wiki](https://code.google.com/p/aseprite/wiki/Home), and
+[contact us](http://groups.google.com/group/aseprite-discuss) about your changes,
+we'll give you editing permissions if they looks fine. Also you can
+make some comments in the Wiki itself.
+
+If you are going to write a Wiki page about some topic, we recommend
+you to take screenshots or record a little GIF showing the steps.
+
+* As screen recording software, on Windows you can generate GIF files
+  using licecap: http://www.cockos.com/licecap/
+* You can upload the PNG/GIF files in http://imgur.com/ temporarily.
+
+## Issues
+
+You can review issues, make comments, or create new ones (features,
+bug reports, etc.) in our
+[issue tracker](http://code.google.com/p/aseprite/issues/list).  You
+are encouraged to create mockups for any issue you see and attach them.
+
+## Hacking
+
+The first thing to keep in main if you want to modify the source code:
+checkout the **dev** branch. It is the branch that we use to develop
+new features and fix issues that are planned for the next big release.
+
+To start looking the source code, see how it is organized in
+[src/README.md](https://github.com/aseprite/aseprite/tree/dev/src/#aseprite-source-code)
+file.
+
+## Get the Source Code
+
+If you want to help in Aseprite, first of all you will need a fresh
+copy of the Git repository. It is located in GitHub, right here:
+
+https://github.com/aseprite/aseprite
+
+You can clone it locally using the following command (read-only URL):
+
+    git clone -b dev git://github.com/aseprite/aseprite.git
+
+On Windows you can use programs like
+[msysgit](http://msysgit.github.io/) to clone the repository.
+
+## Forking & Pull Requests
+
+You can fork the GitHub repository using the Fork button at
+[https://github.com/aseprite/aseprite](https://github.com/aseprite/aseprite).
+
+The Pull Requests (PR) systems works in this way:
+
+1. You've to create a new branch from `dev`, e.g. `fix-8` to fix the issue 8.
+1. Start working on that new branch, and push that branch to your fork.
+1. Create a new PR to merge your `fix-8` branch to official `dev`.
+1. If the PR is accepted, your branch is merged with `dev`.
+1. You will need to pull changes from the official `dev` branch (merge
+   it with your own `dev` branch), and finally you can discard your
+   own `fix-8` branch (those changes are already)
+1. Continue from the new `dev`.
+
+To keep in mind: **always** start working from the `dev` head, if you
+want to fix three different issues, create three different branches
+from `dev` and then send three different PR. Do not chain all the
+fixes in one branch (e.g. `fix-issues-3-and-8-and-25`).
+
+## Contact
+
+Ask for help in the [Aseprite group](http://groups.google.com/group/aseprite-discuss).