Athena guidelines

Here we propose a new "Athena guidelines" page that exist in addition to Scott's pure C++ guidelines. Possible items to include:

• correct use of read, write and decor handles
• not using tools to store or transmit data between components
• not setting up threads internally in algorithms
• use of AthReentantAlgorithm
• other thread unfriendly things
• Doxygen-friendly comment strings (Edit @fwinkl: that's already part of the C++ coding guidelines)
• etc etc