update docs - contrib packages aren't there

[mikofski]

the gen_coeffs and other packages in contrib aren't in the documentation.

[mikofski]

see Trello

[mikofski]

@chetan201 any updates on this issue? it's a real bummer that the contrib packages are in the documentation, especially gen_coeffs. Sorry I meant to add them, but I guess I forgot.

Someone will need to

• add a file to pvmismatch/docs called contributed.rst

• add headers and directives like

  # Contributed
  .. automodule:: pvmismatch.contrib
  
  # Generating Coefficients
  .. automodule:: pvmismatch.contrib.gen_coeffs
     :members:
  

• update index.rst contents section to include contributed

[mikofski]

closed by #89 in commit e733ebb