Add CONTRIBUTING.md to guide contributions.

This contribution guidelines largely based on the guidelines of
the following two projects:

* [bootstrap](https://github.com/twbs/bootstrap)
* [caffe](https://github.com/BVLC/caffe)

Thanks them!

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+Contributing
+================
+
+Issues
+--------
+The GitHub Issues is used to track the development progress, issues,
+bugs, feature requests.
+
+However, the installation and usage related problems should NOT go to
+the Issues, and the Wiki (TODO) or mailing lists (TODO) can be adopted
+for those purposes.
+
+
+Pull Requests
+----------------
+So you already have a patch, an improvement, or even a new feature?
+It's GREAT!  And let's have it!
+
+To make a nice pull request (PR), the following things should be noted:
+
+* Please adhere to the [Code Guidelines](#code-guidelines) used
+  throughout the project, and other requirements (e.g., test).
+* The PR should generally go to the ``master`` branch;
+  the ``gh-pages`` branch should be generated from the documentation
+  source files.
+* It is recommended to use *topic/feature branch* for a PR.
+* A PR should focus on one clear thing, so making multiple smaller PRs
+  is better than making one large PR.
+* A PR should be made of many commits where appropriate, with each commit
+  be a small, atomic change representing one step in the development.
+
+The following procedure is the recommended way to get your work
+incorporated in the project:
+
+1. [Fork](https://help.github.com/fork-a-repo/) the project, clone your
+   fork, and configure the remotes:
+
+   ```sh
+   # Clone your fork of the repo into the current directory
+   git clone https://github.com/<your-username>/fg21sim.git
+   # Navigate to the newly cloned directory
+   cd fg21sim
+   # Assign the original repo to a remote called "upstream"
+   git remote add upstream https://github.com/liweitianux/fg21sim.git
+   ```
+
+2. If you cloned a while ago, get the latest changes from upstream:
+
+   ```sh
+   git checkout master
+   git pull upstream master
+   ```
+
+3. Create a new topic branch (off the main project development branch)
+   to contain your feature, change, or fix:
+
+   ```sh
+   git checkout -b <topic-branch-name>
+   ```
+
+4. Commit your changes in logical chunks. Please adhere to these
+   [git commit message guidelines](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
+   Use Git's [interactive rebase](https://help.github.com/articles/interactive-rebase)
+   feature to tidy up your commits before making them public.
+
+5. Locally merge (or rebase) the upstream development branch into your
+   topic branch:
+
+   ```sh
+   git pull [--rebase] upstream master
+   ```
+
+6. Push your topic branch up to your fork:
+
+   ```sh
+   git push origin <topic-branch-name>
+   ```
+
+7. [Open a Pull Request](https://help.github.com/articles/using-pull-requests/)
+    with a clear title and description against the `master` branch.
+
+
+Code Guidelines
+-------------------
+
+### Python
+
+Adhere to the [PEP 8](https://www.python.org/dev/peps/pep-0008) code style
+and also take a look at this
+[code style](http://docs.python-guide.org/en/latest/writing/style/) from
+[The Hitchhiker's Guide to Python](http://docs.python-guide.org/).
+
+It is strongly recommended to check the code with [``flake8``](https://gitlab.com/pycqa/flake8) which wraps ``pep8``, ``pyflakes``, and other checkers together.
+So make sure it is installed before carrying on.
+
+* Vim users:
+
+  1. Recommend to install the [``spf13-vim``](https://github.com/spf13/spf13-vim)
+     configuration distribution, which already integrates the great
+     [``syntastic``](https://github.com/scrooloose/syntastic) syntax checking
+     plugin;
+
+  2. Add the following recommended settings to ``~/.vimrc.local``:
+
+     ```vim
+     let g:syntastic_always_populate_loc_list = 1
+     let g:syntastic_auto_loc_list = 1
+     let g:syntastic_check_on_open = 1
+     let g:syntastic_check_on_wq = 0
+     ```
+   3. Check syntax manually with command ``:SyntasticCheck``, then browse
+      the found errors/warnings in a subwindow with command ``:Errors``,
+      and jump using commands ``:lnext`` and ``:lprev``.
+
+* Emacs users:
+
+  1. The [``spacemacs``](https://github.com/syl20bnr/spacemacs) is a great
+     configuration distribution to use.
+
+  2. Enable the [``syntax-checking``](https://github.com/syl20bnr/spacemacs/blob/master/layers/syntax-checking/README.org) layer,
+     and you're ready to go.
+
+
+License
+-------
+By contributing your code, you agree to license your contribution under
+the [MIT License](LICENSE).
+
+By contributing to the documentation, you agree to license your contribution
+under the [Creative Commons Attribution 3.0 license](https://creativecommons.org/licenses/by/3.0/us/deed.en_US).