Add documents for community contributions

Author: mebeim

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,7 @@
+<!--
+IMPORTANT: PLEASE READ!
+
+I most likely will not accept pull requests for changes to solutions and walkthroughs. Please read "CONTRIBUTING.md" (https://github.com/mebeim/aoc/blob/master/CONTRIBUTING.md) before submitting a pull request. You can always create an Issue or start a Discussion if you wish.
+
+Thank you for your time and interest in my project.
+-->

CONTRIBUTING.md

@@ -0,0 +1,56 @@
+Contributiung
+=============
+
+Issues & discussions
+--------------------
+
+You can open an issue or discussion for virtually anything you want:
+
+- Report typos or other mistakes.
+- Ask me a question.
+- Discuss about an existing or alternative solution.
+- Propose a change or improvement.
+
+
+Pull Requests
+-------------
+
+Please do *not* submit pull requests for:
+
+- Changes to original solutions: I only keep those as a snapshot of the first
+  solution I wrote when solving the puzzles. They are not really supposed to be
+  changed, and definitely *not* supposed to be clean, optimal or even correct. I
+  only update those if I update some of the used utiliy functions just to keep
+  them at least runnable.
+
+- Changes to clean solutions and walkthrougs: the choices I make for clean
+  solutions are mostly documented in my walkthroughs. If you think that
+  something is off, or that some improvement is possible, you are most welcome
+  to [open an issue][new-issue] or [start a discussion][new-discussion] to tell
+  me about it. I prefer this because (1) it's less work for you, (2) I need to
+  understand and adapt the suggestion before even thinking of applying it, (3)
+  I will also have to adapt the walkthrough at the same time, and finally (4) I
+  might change mind about the license that I am using for my work in the future.
+
+- Grammatical errors, typos, phrasing mistakes in walkthrougs:
+  please, [open an issue][new-issue] instead. I update the repository
+  walkthrougs constantly, and keep track of things that need to get fixed in a
+  private list. Therefore, there is a high chance that I already spotted the
+  problem by myself, and since I try to keep "fix typo" commits to a minimum, I
+  am just waiting to do a single commit with a batch of fixes.
+
+Feel free to open pull requests for:
+
+- Adding new solutions derived from mine: different approaches, alternative
+  solutions using interesting external libraries, etc. If you do, put those
+  under an appropriate folder: `YEAR/misc/dayXX/` and use meaningful file names.
+
+- Editing/fixing/improving anything under the `YEAR/misc/` folders that is not a
+  solution: scripts for visualizations, the script used to fetch leaderboards,
+  etc.
+
+- Editing/fixing/improving anything outside the different year's folders:
+  utilities, documentation (including this document), and so on.
+
+[new-issue]: https://github.com/mebeim/aoc/issues/new
+[new-discussion]: https://github.com/mebeim/aoc/discussions

README.md

@@ -72,6 +72,14 @@ If you enjoy what I'm doing and would like to say "thanks", you can
 [buy me a drink here][paypal-donate-btn].
 
 
+Contributing
+------------
+
+If you have question or spotted a typo/bug/mistake, you are most welcome to
+[submit a new issue][new-issue]. For pull reqeusts and other kinds of
+contributions, please read [`CONTRIBUTING.md`][contributing].
+
+
 Licensing
 ---------
 
@@ -107,6 +115,9 @@ license, which you can find in the file
 [2018-lea]: 2018/misc/full_leaderboard.md
 [2018-cal]: 2018/misc/calendar.gif
 
+[contributing]: /CONTRIBUTING.md
+[new-issue]:    https://github.com/mebeim/aoc/issues/new
+
 [aoc-about]:   https://adventofcode.com/2019/about
 [aoc-eric]:    https://twitter.com/ericwastl
 [aoc-support]: https://adventofcode.com/2019/support