Add jest coverage configuration docs (#3279)

Author: mattphillips

packages/react-scripts/template/README.md

@@ -1374,6 +1374,41 @@ Run `npm test -- --coverage` (note extra `--` in the middle) to include a covera
 
 Note that tests run much slower with coverage so it is recommended to run it separately from your normal workflow.
 
+#### Configuration
+
+The default Jest coverage configuration can be overriden by adding any of the following supported keys to a Jest config in your package.json.
+
+Supported overrides:
+ - [`collectCoverageFrom`](https://facebook.github.io/jest/docs/en/configuration.html#collectcoveragefrom-array)
+ - [`coverageReporters`](https://facebook.github.io/jest/docs/en/configuration.html#coveragereporters-array-string)
+ - [`coverageThreshold`](https://facebook.github.io/jest/docs/en/configuration.html#coveragethreshold-object)
+ - [`snapshotSerializers`](https://facebook.github.io/jest/docs/en/configuration.html#snapshotserializers-array-string)
+
+Example package.json:
+
+```json
+{
+  "name": "your-package",
+  "jest": {
+    "collectCoverageFrom" : [
+      "src/**/*.{js,jsx}",
+      "!<rootDir>/node_modules/",
+      "!<rootDir>/path/to/dir/"
+    ],
+    "coverageThreshold": {
+      "global": {
+        "branches": 90,
+        "functions": 90,
+        "lines": 90,
+        "statements": 90
+      }
+    },
+    "coverageReporters": ["text"],
+    "snapshotSerializers": ["my-serializer-module"]
+  }
+}
+```
+
 ### Continuous Integration
 
 By default `npm test` runs the watcher with interactive CLI. However, you can force it to run tests once and finish the process by setting an environment variable called `CI`.