Created Contribution guide (markdown)

Contribution-guide.md

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+So, you want to help develop Brogue CE? Thank you!
+
+We have a Discord server for contributors; feel free to join it:
+<https://discord.gg/8pxE4j8>
+
+There are some ports of CE. They adopt all our changes, but you can contribute to port-specific parts at their repos:
+
+- Web Brogue: <https://github.com/flend/web-brogue>
+- iPad: <https://github.com/btaylor84/iBrogueCE>
+
+For general help on using GitHub: <https://docs.github.com/>
+
+## How you can help
+
+- **Test** the latest changes on the *master* branch, and let us know what you
+  think and report any bugs you run into! See [[Playing dev versions]].
+
+- **Fix bugs.** See issues labelled *bug*. (Also see *good first issue* for
+  suggested starter fixes.)
+
+- **Implement approved features.** Tickets with the *enhancement* label have
+  been greenlit and just need someone to pick them up!
+
+- **Add detail to ideas and tickets.** Unlabeled but open tickets just need some
+  more attention or discussion to decide what they really mean and whether we
+  want to try them. Turning a vague feature idea into a more detailed
+  description can be useful to figure out edge cases and also helps whoever
+  might implement it.
+
+- **Improve the tiles** or make new alternative tile sets.
+
+## Pull request guide
+
+### Base branch
+
+Brogue CE version numbers follow 1.MINOR.PATCH. Except in rare circumstances,
+gameplay changes are only allowed in minor releases.
+
+PRs should be based on:
+
+* *master* for gameplay changes for the next minor-point release
+* *release* for bug fixes and other non-gameplay changes, for the next patch
+  release. It is merged into *master* periodically.
+
+### Commits
+
+I find a clear Git history very beneficial to work with, so I care quite a bit
+about how commits are presented in PRs.
+
+Please read my [tips for using Git effectively][Git guidance], which can be
+considered guidelines for contributing to this project.
+
+[Git guidance]: http://tmewett.com/git-guidance/
+
+### Change files
+
+When making user-facing changes, please add a non-technical description of each
+change to a .md file in `changes/`. These files are collated to
+create the release notes.
+
+If the change is from one commit, include this file in it. For a branch of
+multiple commits, add it in a separate commit.
+
+## Code style
+
+- Use 4 spaces of indentation.
+- Be consistent with formatting (pay attention to whitespace between brackets,
+  commas, etc.) and try to follow how existing code looks.
+- Declare functions and variables local to a file as `static`.
+- Prefer `int` over `short` in new integer declarations.
+- Some existing code declares all variables at the top of functions, but this isn't necessary to replicate.
+- Use braces for control structures on multiple lines:
+
+  ```c
+  // no
+  if (cond)
+      action();
+
+  // yes
+  if (cond) {
+      action();
+  }
+
+  // acceptable
+  if (cond) action();
+  ```
+
+- When writing bracketed lists over multiple lines, wrap it naturally and don't
+  align to the open bracket (this includes declarations):
+
+  ```c
+  // yes
+  some_function(
+      a, b,
+      c,
+      d
+  );
+
+  // no
+  some_function(a, b,
+                c,
+                d);
+  ```
+
+- When writing multi-line if/while conditions, use at least the same indentation
+  as the body. It can be clearer to over-indent to separate it
+
+  ```c
+  // same indent is ok, but...
+  while ((A && B)
+      || C) {
+      ...
+  }
+
+  // over-indent can be clearer
+  while ((A && B)
+          || C) {
+      ...
+  }
+
+  // a gap works too
+  while ((A && B)
+      || C) {
+
+      ...
+  }
+  ```