In troubleshooting guide, add link to compat checker

[garrettjonesgoogle]

https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/TROUBLESHOOTING.md#alpn-is-not-configured-properly needs to point people to https://github.com/GoogleCloudPlatform/google-cloud-java/tree/master/google-cloud-util/google-cloud-compat-checker , because it's much easier to have a tool tell you what's not compatible than to have to look through the whole list of possible incompatibilities to figure it out.