README: trim down the content
README: trim down the content

This README is huge; the document shouldn't take more than two or three
screens. If we need more docs, they should be linked to and live
elsewhere.

For now, start by removing unnecessary sections. Gitter has been
completely unused since 2017, so remove it. The table of contents also
takes a lot of vertical space, when we don't want to have that much
content to begin with.

Tips on forking are also unnecessary, since that's the same for all Go
projects. Moreover, this is out of date now that the project is a Go
module.

Remove the help output, as that can be obtained extremely easily by just
running 'jira -h'.

Finally, remove the v0-v1 changes, as v0 was last released over three
years ago. It's extremely unlikely that anyone still has to upgrade from
the older version.

Netflix-Skunkworks/go-jira,go-jira/jira,Netflix-Skunkworks/go-jira,go-jira/jira,Netflix-Skunkworks/go-jira