Add warning about HDFStore concurrent reads
Not too much to say; this is a documentation PR.

@yikelu can you update according to above comments?
 @jreback I've been incredibly busy, and I haven't figured out the
documentation.

On Sun, Jan 18, 2015 at 3:31 PM, jreback notifications@github.com wrote:

> @yikelu https://github.com/yikelu can you update according to above
> comments?
> 
> —
> Reply to this email directly or view it on GitHub
> https://github.com/pydata/pandas/pull/7944#issuecomment-70427037.
 @yikelu can you update, pls.
 closing as stale, but if you would like to reopen and fixup pls do.
