Update Sync Gateway / Couchbase Server compatibility section

Describe any compatibility issues between Sync Gateway and CB services

1) XDCR - include link to the section that discusses active active replucation

2) Eventing 

Our current documentation is not very clear on the deployment recommendations when using XDCR with SGW . We have seen an uptick in number of questions related to this topic. 

Although we specify one-directional XDCR as the option , we are not clear that SGW in target cluster must be read-only mode. Also, the impact of convergence and enabling import-docs on the both clusters (that would effectively be write) .

Please refer to this customer escalation . Specifically, read comments from Adam Fraser which elaborates on the various cases and recommendations . 

 (CC Amarantha Kulkarni)