docs(usage): postgres UNIX socket example (#9301)

docs/usage.md

@@ -54,6 +54,9 @@ const sequelize = new Sequelize('database', 'username', 'password', {
 
   // custom host; default: localhost
   host: 'my.server.tld',
+  // for postgres, you can also specify an absolute path to a directory
+  // containing a UNIX socket to connect over
+  // host: '/sockets/psql_sockets'.
  
   // custom port; default: dialect default
   port: 12345,
@@ -202,6 +205,19 @@ const sequelize = new Sequelize('database', 'username', 'password', {
 })
 ```
 
+To connect over a unix domain socket, specify the path to the socket directory
+in the `host` option.
+
+The socket path must start with `/`.
+
+```js
+const sequelize = new Sequelize('database', 'username', 'password', {
+  // gimme postgres, please!
+  dialect: 'postgres',
+  host: '/path/to/socket_directory'
+})
+```
+
 ### MSSQL
 
 The library for MSSQL is`tedious@^1.7.0` You'll just need to define the dialect: