Add service documentation.

docs/intermittent-tracker.md

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+# Intermittent Tracker service
+
+This service runs a flask server that provides a Github webhook endpoint.
+It receives notifications whenever the Github issue tracker is updated,
+and keeps a local database of all open issues that are marked with the
+`I-intermittent` label. This server also allows querying this database,
+so that the `./mach filter-intermittents` command does not need to use
+the Github API every time we run tests on our build machines.
+
+See [the project source code](https://github.com/servo/intermittent-tracker/tree/master/intermittent_tracker) for more information.