add postgres publication guidance (#278)

Author: kobiebotha

snippets/postgres-powersync-publication.mdx

@@ -1,6 +1,13 @@
+
 ```sql
--- Create a publication to replicate tables. 
--- Specify a subset of tables to replicate if required.
--- The publication must be named "powersync"
+-- Create a publication to replicate tables. The publication must be named "powersync"
 CREATE PUBLICATION powersync FOR ALL TABLES;
-```
+```
+
+<Warning>
+Note that the PowerSync Service has to read all updates present in the publication's WAL, regardless of whether the table is referenced in your Sync Streams / Sync Rules definitions. This can cause large spikes in memory usage or introduce replication delays, so if you're dealing with large data volumes then you'll want to specify a comma separated subset of tables to replicate instead of `FOR ALL TABLES`.  
+</Warning>
+
+<Warning>
+The snippet above replicates all tables and is the simplest way to get started in a dev environment.
+</Warning>