README links documentation for master and develop branches

From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2019-07-22 19:28:10

Hi,

Quick note that the README offers links to both docs:
https://github.com/boostorg/gil/blob/develop/README.md

Every time Travis CI builds complete for develop branch,
new the docs are re-generated and deployed for the develop
branch. So, this is useful to know whenever you aim to update the docs,
that the updates will be accessible soon after commits have been merged.

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Second, single huge tutorial section has been split into separate sections:
https://github.com/boostorg/gil/pull/325

Feedback welcome!

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net