[PR] [DOCS] opam 2.x.x is expected for build instructions
[PR] [DOCS] opam 2.x.x is expected for build instructions

Summary:
Walking through the instructions on a Linux machine resulted in an
opam `1.x.x` version from the stable package manager repositories.
Meaning commands like `opam switch create . --deps-only -y` failed.

This PR adds a step to validate we have opam `2.x.x` and ask users
to manually upgrade if necessary.
Pull Request resolved: https://github.com/facebook/flow/pull/8320

Reviewed By: nmote

Differential Revision: D20341825

Pulled By: mroch

fbshipit-source-id: 6c30087654cd03838777cc3e9a321a3ca2d0e68c

mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow