update doc to warn potential partial coverage in watch mode

[connectdotz]

Noticed the coverage sometimes is incorrect in watch mode, depends on what files are changed and their relationship with the target file. It does make sense with the --watch flag that it will only run tests that changed and thus produce partial coverage sometimes. See more detailed discussion in jestjs/jest#1284.

In theory, we can pass --watchAll to ensure the coverage is correct, but it could have a significant performance implication especially for large projects. I don't recall many issues related to this kind of inconsistency, so not sure if it justifies the change/impact. One other possible solution is to introduce a customization flag for using --watchAll with coverage. Should be pretty straightforward if there are enough use cases.

Anyway, I ended up just updated the README to give a warning and corrected a bug I found during the investigation.

[coveralls]

Pull Request Test Coverage Report for Build 878

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.047%

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Totals
Change from base Build 876:	0.0%
Covered Lines:	1144
Relevant Lines:	1254

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💛 - Coveralls