Post pages need explanatory text

[tmtmtmtm]

At the minute they just show a list of people with a fairly meaningless heading:

http://theyworkforyou.github.io/uganda-parliament-watch/post/pwd/10/

[chrismytton]

@tmtmtmtm has pointed out that the text on these pages should be a markdown include, like the footer text, so it can be easily edited in prose.io.

[chrismytton]

I've added the markdown templates in #48, these are in the _includes directory, e.g. _includes/ex_officio.md.