doc(proposal): flaky (#4)

Author: JakobJingleheimer

proposals/feature proposal - flaky.md

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+# Feature proposal: `flaky`
+
+This flag causes a test-case or suite to be re-run a number of times until it either passes or has not passed after the final re-try.
+
+## API
+
+```js
+describe.flaky('some temperamental thing', () => {
+  it('may take several times', () => {…});
+  
+  it('may also take several times', () => {…});
+});
+
+it.flaky('should do something', () => {…});
+```
+
+```js
+describe('some temperamental thing', { flaky: true }, () => {
+  it('may take several times', () => {…});
+  
+  it('may also take several times', () => {…});
+});
+
+it('may take several times', { flaky: true }, () => {…});
+it('may also take several times', { flaky: 100 }, () => {…});
+```
+
+When `flaky` is `true`, the test harness re-tries the suite or test-case up to the default number of times (eg 20), inclusive.
+
+When `flaky` is falsy (the default), the test harness does not re-try the suite or test-case.
+
+When `flaky` is a positive integer, the test harness re-tries the suite or test-case up to the specified number of times, inclusive.
+
+When `flaky` is a non-positive integers or non-boolean, the test harness emits/throws an error.
+
+When both the suite and an included test-case specify the `flaky` flag, the test-case's `flaky` value wins.
+
+## Output
+
+When a suite or test-case is marked `flaky`, the number of re-tries taken to achieve a passing result is reported with that suite or test-case.
+
+The summary also lists the total number of _test-cases_ marked flaky; a "flaky" suite containing 10 test-cases increases the total flaky count by 10.
+
+### Example: TAP
+
+```
+ok 1 - may take several times # FLAKY 42 re-tries
+
+tests 1
+suites 1
+pass 1
+fail 0
+flaky 1
+cancelled 0
+skipped 0
+todo 0
+```
+
+## Notes
+
+Feature documentation (possibly in a Learn article rather than API documentation) warns users of common caveats related to re-tries (e.g. [idempotence](https://en.wikipedia.org/wiki/Idempotence), statefulness).