RFR: Add info about MANPATH to README.

Mon Jun 29 12:05:50 UTC 2020

On Fri, 26 Jun 2020 14:34:53 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> Add info about MANPATH to README.

Thanks @magicus!

Just a very minor nit - the custom is to _not_ end pull request titles with a period (just like we don't end commit
message titles with a period). You can edit the title of this pull request by either clicking the "Edit" button in a
browser or if you are using the Skara CLI tools: `git pr set --title`.

-------------

PR: https://git.openjdk.java.net/skara/pull/680