Document options for babel-generator

[ForbesLindesay]

This adds a table containing the options and a brief description of each one. I plan to use this module separately from babel so it will be useful to have these documented.

[codecov-io]

Current coverage is 84.93%

Merging #3251 into master will decrease coverage by -0.31% as of bd7b202

@@            master   #3251   diff @@
======================================
  Files          215     215       
  Stmts        15699   15699       
  Branches      3360    3360       
  Methods          0       0       
======================================
- Hit          13383   13334    -49
- Partial        674     723    +49
  Missed        1642    1642       

Review entire Coverage Diff as of bd7b202

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[ForbesLindesay]

Unless you're measuring the code coverage of README files, I think that comment is mistaken.

[hzoo]

Thanks for this! Yeah don't worry about the coverage

[jamiebuilds]

Awesome :)