README: Use pull and add --allow-unrelated-histories

README: Use pull and add --allow-unrelated-histories

Using pull gets us down to a single command, leaves no local remote,
and results in a more-descriptive commit message:

  Merge branch 'signed-off-by' of git://tremily.us/signed-off-by

vs. the old:

  Merge branch 'signed-off-by' of s-o-b

--allow-unrelated-histories is required for this sort of thing since
e379fdf3 (merge: refuse to create too cool a merge by default,
2016-03-18, v2.9.0 [1]) and 09c2cb87 (pull: pass
--allow-unrelated-histories to "git merge", 2016-03-18, v2.9.0 [2]).

[1]: https://github.com/git/git/commit/e379fdf34fee96cd205be83ff4e71699bdc32b18
[2]: https://github.com/git/git/commit/09c2cb877a9cc0f6f25eac8d43663f4e8f3309a4

Signed-off-by: W. Trevor King <wking@tremily.us>