DOC: add Hacking guide

HACKING.md

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+# Hacking
+
+## Coding style
+
+This is more or less kernel coding style, but the light version, see
+https://www.kernel.org/doc/Documentation/CodingStyle for a general idea.
+
+Usually you are good to go if you just match the surroundings, but here
+are some more details...
+
+### Random rules:
+* english only
+* no CamelCase
+* opening braces for functions are placed on the next line, opening braces for non-functions (such as loops) are placed on the same line
+* if all conditions of an if, else if, else clause only have one line of instructions, then we omit the scope-operators {...}
+* spaces around stuff like =, +, - etc, as well as keywords (like while, for etc), but not around function names
+* no C99-style comments like // foo
+* 80 char limit
+* no trailing white spaces
+* no implicit function declarations, even for static functions
+
+### Differences to kernel coding style:
+* fine to use tabs with 4 characters instead of 8 (but never whitespaces)
+* newline after return type
+
+## Comments
+
+* comments are in doxygen format
+* all functions, all data types, all macros, all typedefs... basically everything
+* comments usually at declaration, not definition (structs are an exception for example)
+
+## Good practices
+
+* use const modifiers whenever possible, especially on function parameters
+* if unsure whether to make a function static or not, make it static first
+* use unsigned ints instead of signed ints whenever possible
+
+## How to contribute
+
+* pull request on github
+* email with pull request
+* email with patch