Front-End README needs details on yvm

[ecerami]

The current Front-end README file mentions the use of nvm (node version manager), but does not mention the need for yvm (yarn version manager). According to @Luke-Sikina, the following instructions are first needed to get things running:

yvm use 1.21.1
nvm use 8.12.0

then:

export BRANCH_ENV=master
yarn install --frozen-lockfile
yarn run buildDLL:dev
yarn run start

If @alisman agrees, I can try to update the docs.

[Luke-Sikina]

Hey Ethan, just saw this. I made a PR for this already and it looks like it got merged: cBioPortal/cbioportal-frontend#3018