Update README.md with virtualenv information

[dmosorast]

According to the Singer Getting Started guide, taps and targets should be run in separate virtual environments.

In an effort to improve documentation, this should be reflected in the READMEs of the canonical "starter" taps/targets, this repo being one of them.

Requested Change
Adapt the command from the getting started guide to be the first step of this repo's README in a way that works in conjunction with tap-exchangeratesapi (replace tap-fixerio with this one, as it's the current no-configuration tap since Fixer changed their API).

[cgardens]

Should we close this ticket? Seems like the fix was successfully merged.