Readme.md for the design decisions for the call graph analyses

[errt]

Taken from Bitbucket#175

During the development of the pluggable call graph framework, we made some design decisions, like direct vs. indirect callees, incomplete callsites, high soundness mode, various callees properties, the handling of the thread api (mark callers as vm reachable but callees as incomplete). Framework users should have a starting point to get an overview about these decisions.

[bhermann]

Should be done in the new knowledgebase repo.