CONTRIBUTING: Be more detailed in how we prefer contributions.

I have had to ask "why should we change from tool <foo> to <bar>" a few
times now for the project and some of the commit messages seem like they
could have already have answered all these questions beforehand.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>

CONTRIBUTING.md

@@ -1,21 +1,73 @@
-Please, before sending patches, it is helpful if you:
+Please, before sending patches, read these brief comments. They are here to
+help the project have both its users happy using the program and the
+developers/maintainers feel good when trying to change code that other
+people contributed.
 
-* Make sure that you have all dependencies installed, like via
+For the record, when this document mentions "I", it mostly means Rogério
+Brito's (@rbrito) is the one to blame.
 
-        pip install -r requirements.txt
-        pip install -r requirements-dev.txt
+# Write good commit messages
 
-* Run the test suite with
+When you write your pull request and your commit messages, please, be
+detailed, explaining why you are doing what you are doing. Don't be afraid
+of being too verbose here.  Also, please follow the highly recommended
+guidelines on how to write good [good commit messages][commit-msgs].
 
-        py.test coursera/test -v --cov coursera --cov-report html
+When in doubt, follow the model of the Linux kernel commit logs. Their
+commit messages are some of the best that I have seen. Also, the ffmpeg has
+some good messages that I believe that should be followed. If you are in a
+hurry, read the section named
+["Contributing" from subsurface's README][contributing].
 
-* Check for any potential bugs with the help of [`pep8`][pep8],
-  [`pyflakes`][pyflakes], and [`pylint`][pylint].
+[commit-msgs]: https://robots.thoughtbot.com/5-useful-tips-for-a-better-commit-message
+[contributing]: https://github.com/torvalds/subsurface/blob/master/README#L71-L114
+
+
+# Test that your changes don't break existing functionality
+
+Make sure that you have all dependencies installed, like via:
+
+    pip install -r requirements.txt
+    pip install -r requirements-dev.txt
+
+Run the test suite with
+
+    py.test coursera/test -v --cov coursera --cov-report html
+
+If some test fails, please don't send your changes yet. Fix what broke
+before sending your pull request.
+
+If you need to change the test suite, explain in the commit message why it
+needs to be changed (e.g., the page layout or the authentication methods
+from coursera changed, or they implemented a new kind of course).
+
+# Check for potential bugs
+
+Please, help keep the code tidy by checking for any potential bugs with the
+help of [`pep8`][pep8], [`pyflakes`][pyflakes], and [`pylint`][pylint].  If
+you know of any other good tools for analyzing the code, let me know about
+them!
 
 [pep8]: https://pypi.python.org/pypi/pep8
 [pyflakes]: https://pypi.python.org/pypi/pyflakes/
 [pylint]: https://pypi.python.org/pypi/pylint
 
-If you happen to find any issue reported by these programs, we would welcome
-you to fix them, as they are usually very easy to fix and we would all
-benefit from a better code base.
+If you happen to find any issue reported by these programs, I welcome you to
+fix them.  Many of the issues are usually very easy to fix and they are a
+great way to start contributing to this (and other projects in general).
+Furthermore, we all benefit from a better code base.
+
+# Changes in the tools that we use
+
+If you are proposing the use of a substitute of a tool that we already use,
+take a few paragraphs to tell us why we would like to change.
+
+If we are not using something, it is most likely that one of the following
+options applies:
+
+1. I (@rbrito) may know even know that what you are proposing exists or have
+   not yet "seen the light" as to why I should use it instead of using what
+   I am currently using.
+2. Even if I know about the tool, I may not know how to use it, or how it
+   would make me more productive.  Educate me and we all will gain from a
+   better project.