[3.1] various README.md improvements w.r.t. build & branches

[spoonincode]

Various improvements and/or opinionated changes to the README.md

• Document how to do a manual build on Ubuntu 18.04 (resolves Provide documentation on how to get mandel 3.1 compiled on Ubutntu 18.04 #512)
• Warn about high -j values (resolves Provide documentation on how to get mandel 3.1 compiled on Ubutntu 18.04 #512)
• Make clear that main is for development not production and send users over to the releases page to get latest release & tag info; remove the release 3.0 & release 3.1 mentions as that list will just grow forever over time.
• Encourage users to grab the binaries from the release page instead of building.
• Don't use pessimistic language around distro support; be more positive and mention other distros or platforms we're treating "best-effort" even though we don't support them.
• Remove the full-version-label-test|release-build-test|print-build-info-test test exceptions. Day-to-day devs might not want those but when doing a build straight from the repo those tests should pass so let's just simplify this and suggest users to run all them.
• Eliminate any build options & instructions other then the bare minimum. Yes building with Ninja is nice. Yes -DDISABLE_WASM_SPEC_TESTS=yes speeds up build. Yes ccache can be a speed improvement for users who can tolerate it. But all these additions distract from the otherwise simple build instructions that are here to get users bootstrapped easily and simply. Perhaps these could be folded in to an "advanced build options" section or documentation or something.
• Warn users that the non-parallel tests can do bad things like SIGKILL your nodeos.

[stephenpdeos]

Approved