Add code guidelines document.

docs/codestyle.txt

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+Zombie:Reloaded Code Guidelines
+===============================
+
+This is a draft of coding guidelines for Zombie:Reloaded. If something is not
+mentioned here, study the existing code to ensure consistency.
+
+Most of the code was written before we decided on the code style, so we may even
+break our own rules according to this document.
+
+Highly recommended book:
+Clean Code: A Handbook of Agile Software Craftsmanship (2008)
+Robert C. Martin
+
+* Curly braces on separate lines.
+* Indent with 4 spaces instead of tabs (configure your editor).
+* Unix line endings.
+* Always use curly braces with control statements (if, for, while, do).
+* UTF-8 file encoding without BOM.
+* Extract code into multiple functions if code is too complex. ZR is still bad
+  at following this rule.
+* If the code needs comments, it should be refactored/renamed to better explain
+  what it's doing. ZR has a lot of unnecessary comments, mostly because it's not
+  following the single purpose-function rule above.