Add some documentation to the Travis script.

[mikejcurry]

This PR is to add some documentation to the Travis build script based on the discussion in #699.

I’ve just put some information in the script and I haven’t gone into a huge amount of detail. The main thing I wanted to capture was the rationale for building the JVM twice rather than once - which makes perfect sense once you understand the coverage tool; but confused me at first.

Happy to restructure or move elsewhere if that is preferred.

[codecov-io]

Current coverage is 84.42%

Merging #701 into master will not affect coverage as of c11d54f

@@            master    #701   diff @@
======================================
  Files          162     162       
  Stmts         2228    2228       
  Branches        74      74       
  Methods          0       0       
======================================
  Hit           1881    1881       
  Partial          0       0       
  Missed         347     347       

Review entire Coverage Diff as of c11d54f

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[ceedubs]

Wow great. Thanks! 👍

[non]

👍 This is great, thanks so much!

[ghost]

Bit late but nice :)