Add README for the Reporting API in WPT

Change-Id: Ia83f04680f54099997d0ae71cda025ad773e2868
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3421157
Reviewed-by: Rodney Ding <rodneyding@google.com>
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/main@{#964455}

reporting/README.md

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+Tests for the [Reporting API](https://w3c.github.io/reporting/).
+
+The tests in this directory validate the generic functionaity of the Reporting
+API. Since reports are not actually generated by that specification, these tests
+occasionally make use of other integrations, like CSP or Permissions Policy.
+
+## Testing integration with the Reporting API
+
+More comprehensive tests for other specifications' generated reports should be
+in those specs' respective directories.
+
+There are two general methods of testing reporting integration:
+
+* The simpler is with the ReportingObserver interface, generating reports
+  within a document and reading them from script running in that document.
+* For reports which cannot be observed from a document, there is a reporting
+  collector provided which can receive reports sent over HTTP and then serve
+  them in response to queries from the test script. See resources/README.md for
+  details.