CONTRIBUTING.md missing

[giovanism]

Add guidelines for issues, commit messages, and PRs.

So #3 won't happen again

Reference

https://github.com/blog/1184-contributing-guidelines
https://github.com/coala/coala/blob/master/CONTRIBUTING.rst

Proposal

1. Source code (python)

   • Passed pylint source code analyzer (except C0103 for the time being)
   • Maximum line length is set to 79
   • File naming format (Lab)0[1-99]_[A-F](\+[A-F])?(_s[1-9])?(\.py)
   • ...

2. Problem sets

   • Include only the necessary instruction and hint to complete the problem
   • Verbatim with necessary formatting adjustment
   • A footer
   • File naming format (Lab)0[1-99]_[A-F](\+[A-F])?(\.md)
   • ...

3. Commit messages

   • First line < 50 chars
   • ...

4. PRs

5. Issues

[laymonage]

Yep, we need CONTRIBUTING.md in this repo. I like your proposal so far.
Some points I can think of right now:

• For PyLint, we have a problem in class F's turtle solutions since they don't have any turtle object instantiation, which leads to an E1101 error. We're gonna use CI, and I think we can lint those solutions individually in CI (I'm not a fan of adding lint exceptions inside source codes).
• Specify footer format for problem sets.
• Add details about how images should be included.
• Commit messages should use Sentence case. See previous commits for examples.
• PRs' rules are pretty much the same as commit messages', but they're more flexible (I think?).

[giovanism]

@laymonage , I think I can change class F solution to conform with PyLint's rule. Another thing to mention, would it be better for python source code filename format in lower case, to conform PyLint module name rule?

[laymonage]

I didn't know PyLint has rules for module names. Well, if you think so, feel free to send a PR that fixes it.