Provide top level CMake configuration and clear build instructions

[dvrogozh]

Project exposes multiple things:

• PTI library
• Tools (such as unittrace)
• Samples

However, it's unclear how to build some of the above. Product instructs only to build samples leaving instructions for PTI library and tools behind the scene:

pti-gpu/README.md

Line 122 in a928b93

cd <pti_root>/samples/<sample_root>

Can you, please, consider to make the following changes:

1. Flatten CMake configuration by exposing top level CMakeLists.txt file which will allow to build everything at once?
2. Adjust documentation by:
   1. Describing this build entry point
   2. Describing available build options to select build targets
   3. Describing dependencies paying attention to any which are closed source dependencies

CC: @mschilling0, @jfedorov, @frenchwr