docs: add info about convenience maven project

[zmiklank]

This PR proposes docs information about minimal Maven build of the pgjdbc.

[davecramer]

I just built a jar from the current source using gradle and from the source tar and extracted the class files to the filesystem then ran diff -qr postgresql-42.3.2-jdbc-src/ postgresql-42.3.2-SNAPSHOT-jdbc-src/

postgresql-42.3.2-jdbc-src/ postgresql-42.3.2-SNAPSHOT-jdbc-src/
Only in postgresql-42.3.2-jdbc-src/: dependency-reduced-pom.xml
Files postgresql-42.3.2-jdbc-src/src/main/java/org/postgresql/util/DriverInfo.java and postgresql-42.3.2-SNAPSHOT-jdbc-src/src/main/java/org/postgresql/util/DriverInfo.java differ
Files postgresql-42.3.2-jdbc-src/src/main/resources/META-INF/MANIFEST.MF and postgresql-42.3.2-SNAPSHOT-jdbc-src/src/main/resources/META-INF/MANIFEST.MF differ
Only in postgresql-42.3.2-jdbc-src/: target

This seems to be reasonably expected.

I think the checkerframework stuff is removed but that does not effect functionality