Move content from README.txt to Readme.md

[astrogeco]

Is your feature request related to a problem? Please describe.
Having two files named Readme is confusing. Making users click over to a second file when they might expect Readme.md to have a thorough description of the tool is inefficient.

Describe the solution you'd like
Absorb Readme.txt into Readme.md

Describe alternatives you've considered
Have full blown documentation folder using doxygen or similar

Additional context
None

Requester Info
Gerardo E. Cruz-Ortiz