Document Istanbul check-coverage cli command (#834)

Author: cgewecke

README.md

@@ -123,6 +123,7 @@ Common problems & questions:
 + [Running out of stack][1002] (Stack too deep)
 + [Running out of memory][5]
 + [Running in parallel (in CI)][1003]
++ [Running coverage threshold checks][1004]
 
 ## Example reports
 + [metacoin][9] (Istanbul HTML)
@@ -218,4 +219,5 @@ $ yarn
 [1001]: https://docs.soliditylang.org/en/v0.8.0/using-the-compiler.html#input-description
 [1002]: https://github.com/sc-forks/solidity-coverage/blob/master/docs/faq.md#running-out-of-stack
 [1003]: https://github.com/sc-forks/solidity-coverage/blob/master/docs/advanced.md#parallelization-in-ci
+[1004]: https://github.com/sc-forks/solidity-coverage/blob/master/docs/advanced.md#coverage-threshold-checks
 

docs/advanced.md

@@ -138,6 +138,19 @@ There's a nice example of this being done in CircleCI [at Synthetix, here][31].
 
 :bulb: **Pro Tip**: Codecov CI will automatically combine coverage reports sent to them as a batch - if you're using that service you don't need to do this yourself.
 
+## Coverage threshold checks
+
+Istanbul has a command line utility which can be used to set thresholds for different coverage categories and throw an error if your coverage drops below them. (Istanbul is a solidity-coverage dependency so you shouldn't need to install it separately.)
+
+```shell
+# Usage
+
+$ npx istanbul check-coverage ./coverage.json --statements 99 --branches 94 --functions 99 --lines 99
+
+ERROR: Coverage for statements (60%) does not meet global threshold (99%)
+ERROR: Coverage for lines (60%) does not meet global threshold (99%)
+ERROR: Coverage for functions (66.67%) does not meet global threshold (99%)
+```
 
 [22]: https://github.com/JoranHonig/vertigo#vertigo
 [23]: http://spideruci.org/papers/jones05.pdf