Add CI information to CONTRIBUTING.md (#778)

Add information about how the continuous integration is set up in Dendrite and how to run the tests locally so that people don't need to wait around for things to churn.

CONTRIBUTING.md

@@ -20,6 +20,39 @@ should pick up any unit test and run it). There are also [scripts](scripts) for
 [linting](scripts/find-lint.sh) and doing a [build/test/lint
 run](scripts/build-test-lint.sh).
 
+## Continuous Integration
+
+When a Pull Request is submitted, continuous integration jobs are run
+automatically to ensure the code builds and is relatively well-written. Checks
+are run on [Buildkite](https://buildkite.com/matrix-dot-org/dendrite/) and
+[CircleCI](https://circleci.com/gh/matrix-org/dendrite/).
+
+If a job fails, click the "details" button and you should be taken to the job's
+logs.
+
+![Click the details button on the failing build step](docs/images/details-button-location.jpg)
+
+Scroll down to the failing step and you should see some log output. Scan
+the logs until you find what it's complaining about, fix it, submit a new
+commit, then rinse and repeat until CI passes.
+
+### Running CI Tests Locally
+
+To save waiting for CI to finish after every commit, it is ideal to run the
+checks locally before pushing, fixing errors first. This also saves other
+people time as only so many PRs can be tested at a given time.
+
+To execute what Buildkite tests, simply run `./scripts/build-test-lint.sh`.
+This script will build the code, lint it, and run `go test ./...` with race
+condition checking enabled. If something needs to be changed, fix it and then
+run the script again until it no longer complains. Be warned that the linting
+can take a significant amount of CPU and RAM.
+
+CircleCI simply runs [Sytest](https://github.com/matrix-org/sytest) with a test
+whitelist. See
+[docs/sytest.md](https://github.com/matrix-org/dendrite/blob/master/docs/sytest.md#using-a-sytest-docker-image)
+for instructions on setting it up to run locally.
+
 
 ## Picking Things To Do