CONTRIBUTING.md: how pull requests avoid editing the same lines

CONTRIBUTING.md

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+# Contributing to NewsCache
+
+Changes reach `main` only through pull requests; the required CI checks
+must pass. Several pull requests are often open at once, so please keep
+them from editing the same lines:
+
+- One concern per pull request, on a branch from `main` (for example
+  `fix/<topic>`). Don't collect unrelated fixes on one branch.
+- Don't edit `NEWS` or `debian/changelog`; only the release pull
+  request does. Add your NEWS entry as
+  `changes/<branch name with / as ->.md`, with the NEWS section as its
+  first line (see [changes/README.md](changes/README.md)). The release
+  pull request moves the entries into NEWS.
+- Don't edit `REVIEW.md` in a fix; it's updated in batches after merges.
+- Edit `README.md` only for something users see there: a new option,
+  build dependency or command.
+- Add a test that fails without your fix where that's practical.
+  Don't loosen a test's bounds or checks to make it pass on a slow
+  machine; find out what it really measures.
+- Bring your branch up to date with `main` (merge or rebase) when it
+  no longer merges cleanly.
+
+Don't commit to `trunk`: it mirrors the old Subversion repository.