add tip on wrong jar file usage (#661)

Author: geekpete

docs/troubleshooting.asciidoc

@@ -160,3 +160,14 @@ Consider updating to a newer version of that library.
 
 That mostly concerns JDBC drivers.
 Updating them to a more recent version should resolve the problem.
+
+[float]
+[[trouble-shooting-incorrect-manual-jar-file]]
+==== Failed to find Premain-Class manifest attribute ====
+
+If you are using a manual setup with a `-javaagent` flag against an application server and are seeing the 
+`Failed to find Premain-Class manifest attribute` error and a failure to start, then you might be pointing
+at the incorrect jar file.
+
+The correct jar file to be pointing at should be in the form of `elastic-apm-agent-<version>.jar` and 
+further information about how to download this file can be found <<setup-javaagent, in the manual setup instructions.>>