manpages: add editing instructions to ERB template

Closes #102.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Library/Homebrew/manpages/brew.1.md.erb

+<%
+# To make changes to this man page:
+#
+# - For changes to a specific command (appears in the `COMMANDS` section):
+#   - Edit the top comment in `Library/Homebrew/cmd/<command>.{rb,sh}`.
+#   - Make sure to use the line prefix `#:` for the comments to be recognized as
+#     documentation. If in doubt, compare with already documented commands.
+# - For other changes: Edit this file.
+#
+# When done, regenerate the man page and its HTML version by running `brew man`.
+%>
 brew(1) -- The missing package manager for OS X
 ===============================================