docs: add rebase.conflict-resolver to README

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: abersnaze

README.rst

@@ -143,6 +143,24 @@ options:
   ``PyGitUp`` will show the hashes of the current commit (or the point
   where the rebase starts) and the target commit like ``git pull`` does.
 
+-  ``git-up.rebase.conflict-resolver [cmd]``: If set, ``PyGitUp`` will
+   invoke this command when a rebase conflict occurs. The command should
+   contain a ``{prompt}`` placeholder, which will be replaced with a
+   shell-escaped prompt containing conflict context (branch names,
+   conflicted files, and resolution steps). The command is expected to
+   resolve all conflicts, stage the files, and run
+   ``git rebase --continue``. If it exits with code 0 and the rebase is
+   complete, ``git up`` continues to the next branch. If it fails, the
+   repo is left in the conflicted state for manual resolution.
+
+   Environment variables ``GITUP_BRANCH``, ``GITUP_TARGET``, and
+   ``GITUP_REPO_PATH`` are also set for the resolver process.
+
+   Examples::
+
+     git config git-up.rebase.conflict-resolver "claude -p {prompt}"
+     git config git-up.rebase.conflict-resolver "aider --message {prompt}"
+
 New in v1.0.0:
 ~~~~~~~~~~~~~~