Add gradle build info.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1573803 13f79535-47bb-0310-9956-ffa450edef68
Remove unused dependency from log4j-api.

  - Commons HTTP client isn't used anywhere.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1573805 13f79535-47bb-0310-9956-ffa450edef68
Bump SLF4J to 1.7.6.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1573806 13f79535-47bb-0310-9956-ffa450edef68
Fix typo in migration documentation.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1573808 13f79535-47bb-0310-9956-ffa450edef68