Convert the MongoDB README to markdown to improve readability.

Original Description:

Description

Opened a PR to improve the MongoDB documentation readme: https://github.com/mongodb/mongo/pull/1406

Was told that in order to publish the PR, I needed to:

1. Sign our Contributor's Agreement. (done)

1. Create a new SERVER ticket on JIRA. (this ticket)

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)