Rename the "contrib" directory to "completion".

The original "contrib" name is lousy. Everything in notmuch has been
contributed, and we are integrating as much of it as possible, (rather
than making users grub through contrib looking for useful pieces to
install).

Meanwhile, the only things we have in contrib are command-line
completion scripts, so "completion" makes more sense as a name, (and
helps make "./configure" slightly less ambiguous).

completion/README

@@ -0,0 +1,10 @@
+notmuch completion
+
+This directory contains support for various shells to automatically
+complete partially entered notmuch command lines.
+
+notmuch-completion.bash	Command-line completion for the bash shell
+
+notmuch-completion.tcsh	Command-line completion for the tcsh shell
+
+notmuch-completion.zsh	Command-line completion for the zsh shell