Add PR message expectations (#295)

* Add PR message expectations

* Fix indentation in development guidelines

---------

Co-authored-by: lochhh <changhuan.lo@ucl.ac.uk>

Author: adamltyson

docs/source/get-involved/development_guidelines.md

@@ -58,9 +58,12 @@ one operating system.
 
 * We will use [semantic versioning](https://semver.org/) for all projects.
 
-## Pull requests
+## Pull requests (PRs)
 - Please submit _draft_ pull requests as early as possible (you can still push to the branch once submitted) to
   allow for discussion.
+- Please ensure that the title of your PR clearly explains its purpose. The "why" is more important than the "how"
+  (which should be clear from the code and the PR description). We do not use any standards such as 
+  [Conventional Commits](https://www.conventionalcommits.org) so PR titles should not include this metadata.
 - One approval of a PR is enough for it to be merged.
 - Unless someone approves the PR with optional comments, the PR can be immediately merged by the approving reviewer.
 - Please merge via "Squash and Merge" on GitHub to maintain a clean commit history.