Update README.md with proper install instructions

[dragancla]

That npm install rp-mocha-reporter seems to install a module that hasn't been updated in 1 year and which does not support ReportPortal v5. Should be npm install reportportal/agent-js-mocha#master but, doing so will cause mocha to throw an "rp-mocha-reporter" reporter not found error. I'm guessing it's not the correct name or the reporter function is not exported correctly.

[dragancla]

The readme altogether is pretty misleading since the repo is called agent-js-mocha, the package is called rp-mocha-reporter and the name of the reporter, accoding to the readme, is reporter: 'mocha-rp-reporter' while, in another issue, someone referred to this as mocha-js-reporter

[ElenaRomanchuk]

We have just released the new version of mocha reporter as a new package @reportportal/agent-js-mocha: https://www.npmjs.com/package/@reportportal/agent-js-mocha.
It's fully compatible with ReportPortal version 5.* and async API v2 and has some additional features.
The package rp-mocha-reporter is deprecated now.
We updated readme and repository name with the new version and the new package.
Thanks.