add a CONTRIBUTING file

CONTRIBUTING.md

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+# How to Contribute
+
+incorporeal-cms is a personal project seeking to implement a simpler, cleaner form of what would
+commonly be called a "CMS". I appreciate any help in making incorporeal-cms better.
+
+## Opening Issues
+
+Issues should be posted to my Gitea instance at
+<https://git.incorporeal.org/bss/incorporeal-cms/issues>. I'm not too picky about format, but I
+recommend starting the title with "Improvement:" or "Bug:" so I can do a high level of
+prioritization.
+
+## Providing Code
+
+Some guidelines:
+
+* Code:
+    * Keep in mind that I strive for simplicity in the software. It serves files and renders
+      Markdown, that's pretty much it. Features around that function are good; otherwise, I need
+      convincing.
+    * Follow the style precedent set in the code. Do **not** use Black, or otherwise reformat existing
+      code. I like it the way it is and don't need a militant tool making bad decisions about what is
+      readable.
+    * `tox` should run cleanly, of course.
+    * Almost any change should include unit tests, and also functional tests if they provide a feature
+      to the CMS functionality. For defects, include unit tests that fail on the unfixed codebase, so I
+      know exactly what's happening.
+* Commits:
+    * Squash tiny commits if you'd like. I prefer commits that make one atomic conceptual change
+      that doesn't affect the rest of the code, assembling multiple of those commits into larger
+      changes.
+    * Follow something like [Chris Beams'](https://chris.beams.io/posts/git-commit/) post on
+      formatting a good commit message.
+
+### Contributing
+
+I don't expect contributors to sign up for my personal Gitea in order to send contributions, but it
+of course makes it easier. If you wish to go this route, please sign up at
+<https://git.incorporeal.org/bss/incorporeal-cms> and fork the project. People planning on
+contributing often are also welcome to request access to the project directly.
+
+Otherwise, contact me via any means you know to reach me at, or <bss@incorporeal.org>, to discuss
+your change and to tell me how to pull your changes.
+
+Accepted changes remain the copyright of the original author, but please include appropriate contact
+methods in the event I choose to provide the project under a new license and need to contact you
+to approve the new license terms. Please note that the software is provided under GPLv2 or later.