Duplicate parts of the docs - need some sort of include

[audreyfeldroy]

These files are almost the same, but diverging due to duplication:

• docs/index.rst and some of README.rst
• docs/contributors.rst and CONTRIBUTORS.txt

Let's see what we can do to avoid the duplication:

• Look into RST includes
• Look into the Markdown equivalent of RST includes. If it's better, maybe we'll move from RST to Markdown

[ryancheley]

The docs were converted from rst to md in commit 19f0114