README: Document pre-meeting agenda alteration

On Sat, Aug 08, 2015 at 08:23:21AM -0400, Vincent Batts wrote [1]:
> We said the topics would be proposed on this list. Discussion notes have
> been shared as a Google doc so far. I'm not opposed to markdown notes, but
> initially that just seems like an repo to me.

While touching this paragraph, I also re-wrapped it to match
README.md#markdown-style.

[1]: https://groups.google.com/a/opencontainers.org/d/msg/dev/41jZ9Qe7R5c/ZInajC_0DgAJ
     From: Vincent Batts
     Subject: Re: Open Container Weekly Meeting - Aug 5, 2015
     Date: Sat, 8 Aug 2015 08:23:21 -0400
     Message-ID: <CAN6Zp5yFX8yLG3b-82SAq7AmCxVpoy1tyt0K1ijFqSsCjKPRpg@mail.gmail.com>
     Cc: dev <dev@opencontainers.org>, …

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

README.md

@@ -64,8 +64,9 @@ the `LICENSE` file of this repository.
 ## Weekly Call
 
 The contributors and maintainers of the project have a weekly meeting Wednesdays at 10:00 AM PST.
-Everyone is welcome to participate in the call.  The link to the call will be posted on the mailing
-list each week along with set topics for discussion.
+Everyone is welcome to participate in the call.
+The link to the call will be posted on the mailing list each week along with set topics for discussion.
+Everyone is welcome to propose additional topics or suggest other agenda alterations via the mailing list.
 Minutes for the call will be posted to the mailing list for those who are unable to join the call.
 
 ## Markdown style