docs: explaining clearly how to install non latest binary + toc added

[j-d-carmichael]

• There were no clear sections in the readme on installing mongo above the latest version, eg 4.2.8 on Linux Mint.
• There were no table of contents making the docs hard to jump through, doctoc added for automatically building the main readme

[codecov]

Codecov Report

Merging #321 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #321   +/-   ##
=======================================
  Coverage   78.81%   78.81%           
=======================================
  Files           9        9           
  Lines         925      925           
  Branches      257      263    +6     
=======================================
  Hits          729      729           
  Misses        195      195           
  Partials        1        1           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d405997...822325c. Read the comment docs.

[nodkz]

Thanks a lot!

Especially for DocToc 👍
I didn't know this tool.