[docs] Add interrogate configuration

[simoncozens]

interrogate is a handy tool for checking documentation coverage. This configures interrogate with some project-specific defaults.

[anthrotype]

No idea how this tool works, but I am not convinced that documentation coverage ought to be checked automatically. I don't think that all functions/classes/methods in a project are meant to be exported as public and as such have a docstring attached... Internal-only stuff may well have no docstring or if anything some inline comments for dev use only, but not users of the library.

[simoncozens]

I don't plan to run it automatically. The configuration is just about configuring the tool so that it can be run manually for reporting and finding areas where more documentation is needed. In fact, the point of this configuration is to hide the internal stuff. e.g. without specifying this config:

= Coverage for /Users/simon/others-repos/fonttools/Lib/fontTools/ttLib/tables/ =
------------------------------ Detailed Coverage -------------------------------
| Name                                                |                 Status |
|-----------------------------------------------------|------------------------|
| C_P_A_L_.py (module)                                |                 MISSED |
|   table_C_P_A_L_ (L13)                              |                 MISSED |
|     table_C_P_A_L_.__init__ (L18)                   |                 MISSED |
|     table_C_P_A_L_.decompile (L25)                  |                 MISSED |
|     table_C_P_A_L_.compile (L74)                    |                 MISSED |
|     table_C_P_A_L_.toXML (L159)                     |                 MISSED |
|     table_C_P_A_L_.fromXML (L204)                   |                 MISSED |
|   Color (L236)                                      |                 MISSED |
|     Color.hex (L238)                                |                 MISSED |
|     Color.__repr__ (L241)                           |                 MISSED |
|     Color.toXML (L244)                              |                 MISSED |
|     Color.fromHex (L249)                            |                 MISSED |
|     Color.fromRGBA (L259)                           |                 MISSED |
|-----------------------------------------------------|------------------------|

With this PR:

= Coverage for /Users/simon/others-repos/fonttools/Lib/fontTools/ttLib/tables/ =
------------------------------ Detailed Coverage -------------------------------
| Name                                               |                  Status |
|----------------------------------------------------|-------------------------|
| C_P_A_L_.py                                        |                         |
|   table_C_P_A_L_ (L13)                             |                  MISSED |
|     table_C_P_A_L_.__init__ (L18)                  |                  MISSED |
|   Color (L236)                                     |                  MISSED |
|     Color.hex (L238)                               |                  MISSED |
|     Color.__repr__ (L241)                          |                  MISSED |
|     Color.fromHex (L249)                           |                  MISSED |
|     Color.fromRGBA (L259)                          |                  MISSED |
|----------------------------------------------------|-------------------------|

[anthrotype]

gotcha, thanks for the clarification.