Add development guide with tips and tricks

We could use a simple guide file for new contributors. We have that for git, but not for Python and C++.

• usage of asdf to break the CI build intentionally
• how to add a new feature (e.g. adding a demo so you have a "new" main() function)
• common C++ mistakes (let's see if we can get those in under 100MiB :P)
• redirection to the code_style
• how to profile (python, cython, c++)
  • python (openage.util.profiler.Profiler)
  • cython (activate profiling in cython code, then start profiling with the openage.util.profiler.Profiler)
  • C++ (hit F12 to start and stop profiling, then you'll get a report-PDF)
• summary of
  • useful openage functions (incl redirection to testing/demo docs and others)
    • ENSURE macro
    • log system introduction
    • error::Error and MSG usage
    • libopenage/util/fslike introduction
    • libopenage/util/timer
    • libopenage/util/strings
  • make people aware of existing custom datastructures
    • PairingHeap
    • ...
  • how to add cython files
  • how to add c++ files
  • how to add python files

Start by locating the existing git guide, the code_style documentation, and the testing/demo docs referenced in the issue. Use those as the structure for a contributor development guide covering the unchecked asdf, feature, C++, profiling, utility, data-structure, and file-creation topics. Done means the guide addresses the listed items and links to the existing profiling and style guidance.