Update README.rst

[Jylpah]

I propose adding clarification of the abbreviations in the coverage report. These are not mentioned anywhere in the documentation and there is even a StackOverflow question about those.

===== Addition to section "Usage" under the report ========

Where,

• Stmts refers to the number of statements in your code.
• Miss refers to the number of statements that have not been run.
• Cover is test coverage, or (Stmts - Miss) / (Stmts) * 100.

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I can understand these are "obvious" for the experienced programmers, but maybe not for someone taking a code coverage tool into use for the first time.

I understand you want to keep the documentation short and to the point (it is).