Improve docs to release with Bintray

[jvican]

The page that explains how to release with Bintray is less specific than the one to release with Sonatype.

Concretely, it misses the following sections:

• You don't have a Bintray account.
• You have a Bintray account.
• Clear "Meta info".

The current "Releasing to Maven Central" section should be a sub-section of "You have a bintray account".

[laughedelic]

I think it should also mention that you may want to set some sbt-bintray keys and link to its documentation.

[jvican]

Another data point that is really important: this plugin should explain the appropriate place to define bintray settings. Their defaults are defined in ThisBuild, so it's likely that users define them in Global along with the rest of the publishing settings and sbt doesn't pick these redefinitions.

The bintray settings I'm referring to are bintrayOrganization, bintrayRepository, et al.