add style document

style.md

@@ -0,0 +1,34 @@
+## Callisto code style
+
+C, Lua and makefile code should use tabs for indentation; this allows
+one to set their own tab width preference in their editor. However,
+when code has to be lined up over multiple lines, it should be lined
+up according to 4-width indentation.
+
+C code in Callisto should follow the OpenBSD style(9) document:
+ - http://man.openbsd.org/style.9
+
+Code in the Nix flake should follow the nixpkgs style guide:
+ - https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#code-conventions
+
+## Commits
+
+Prepend your commit message with the name of the Lua library that your
+commit primarily changes, for example if I were to add a new function
+named `copy` to the `fs` library, I might use the commit message
+"fs: add new function 'copy'".
+If the commit does not change code part of a Lua library but still
+changes code, pick a prefix relevant to the code you changed,
+e.g. "tests: add test for fs.copy", "makefile: fix build on NetBSD",
+"flake: remove darwin from platforms".
+If the commit does not change code at all scrap the prefix.
+
+Additionally follow these conventions taken from nixpkgs:
+
+- Create a commit for each logical unit.
+
+- Check for unnecessary whitespace with `git diff --check` before committing.
+
+- If you have commits `pkg-name: oh, forgot to insert whitespace`: squash commits in this case. Use `git rebase -i`.
+
+- For consistency, there should not be a period at the end of the commit message's summary line (the first line of the commit message).