Update contributing guide re merging and CI

Lately we have had a bunch of new devs bumping PRs trying to get things
merged. Everyone gets excited, no need to be too harsh on them. At least
we can try to explain why we might be taking so long.

We (*cough* Andrew) runs a custom CI box that leads to slightly unusual
merge behaviour in this repo if one is unaware of it.

Document our CI better and ask folk not to bump too much.

While we are at it remove the `API changes` section because we removed
the API files already.

CONTRIBUTING.md

@@ -16,6 +16,7 @@ changes to this document in a pull request.
 - [Contribution workflow](#contribution-workflow)
   * [Preparing PRs](#preparing-prs)
   * [Peer review](#peer-review)
+  * [CI and Merging](#merging)
   * [Repository maintainers](#repository-maintainers)
 - [Coding conventions](#coding-conventions)
   * [Naming conventions](#naming-conventions)
@@ -160,8 +161,20 @@ running `just check-api`.
 Check the [API text files](api/README.md) for more information
 on how to install the dependencies and create the text files.
 
+### CI and Merging
+
+We use GitHub for CI as well to test the final state of each PR.
+
+Also we use a local CI box which runs a large matrix of feature combinations as
+well as testing each patch in a PR. This box is often very backlogged, sometimes
+by multiple days. Please be patient, we will get to merging your PRs when the
+backlog clears.
+
 ### Repository maintainers
 
+Like all open source projects our maintainers are busy. Please take it easy on
+them and only bump if you get no response for a week or two.
+
 Pull request merge requirements:
 - all CI test should pass,
 - at least one "accepts"/ACKs from the repository maintainers