Update contributing guide (#1196)

.github/PULL_REQUEST_TEMPLATE.md

@@ -14,6 +14,11 @@ If an item doesn't apply to your pull request, **check it anyway** to make it ap
 If your pull request is a documentation fix or a trivial typo, feel free to delete the whole thing.
 -->
 
+- [ ] Do **not** open pull requests from your `main` branch – **use a separate branch**!
+
+  There's a ton of footguns waiting if you don't heed this warning. You can still go back to your project, create a branch from your main branch, push it, and open the pull request from the new branch.
+
+  This is not a pre-requisite for your your pull request to be accepted, but **you have been warned**.
 - [ ] Added **tests** for changed code.
   Our CI fails if coverage is not 100%.
 - [ ] New features have been added to our [Hypothesis testing strategy](https://github.com/python-attrs/attrs/blob/main/tests/strategies.py).
@@ -28,7 +33,7 @@ If your pull request is a documentation fix or a trivial typo, feel free to dele
           The first number represents the current year.
           So if the current version on PyPI is 22.2.0, the next version is gonna be 22.3.0.
           If the next version is the first in the new year, it'll be 23.1.0.
-- [ ] Documentation in `.rst` files is written using [semantic newlines](https://rhodesmill.org/brandon/2012/one-sentence-per-line/).
+- [ ] Documentation in `.rst` and `.md` files is written using [semantic newlines](https://rhodesmill.org/brandon/2012/one-sentence-per-line/).
 - [ ] Changes (and possible deprecations) have news fragments in [`changelog.d`](https://github.com/python-attrs/attrs/blob/main/changelog.d).
 - [ ] Consider granting [push permissions to the PR branch](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork), so maintainers can fix minor issues themselves without pestering you.