Add configuration docs to Postgresql input plugin (#2515)

* Add configuration docs to Postgresql input plugin

Add configuration docs to PostgreSQL input plugin README (mostly from the source code) though I've not included the configuration example that seems to use all he connections on the database[1].

[1] #2410

* Fix typo in readme and sampleConfig string.

plugins/inputs/postgresql/README.md

@@ -29,3 +29,25 @@ _* value ignored and therefore not recorded._
 
 
 More information about the meaning of these metrics can be found in the [PostgreSQL Documentation](http://www.postgresql.org/docs/9.2/static/monitoring-stats.html#PG-STAT-DATABASE-VIEW)
+
+## Configruation
+Specify address via a url matching:
+
+  `postgres://[pqgotest[:password]]@localhost[/dbname]?sslmode=[disable|verify-ca|verify-full]`
+
+All connection parameters are optional. Without the dbname parameter, the driver will default to a database with the same name as the user. This dbname is just for instantiating a connection with the server and doesn't restrict the databases we are trying to grab metrics for.
+
+A  list of databases to explicitly ignore.  If not specified, metrics for all databases are gathered.  Do NOT use with the 'databases' option.
+
+  `ignored_databases = ["postgres", "template0", "template1"]`
+
+A list of databases to pull metrics about. If not specified, metrics for all databases are gathered.  Do NOT use with the 'ignored_databases' option.
+
+  `databases = ["app_production", "testing"]`
+
+### Configuration example
+```
+[[inputs.postgresql]]
+  address = "postgres://telegraf@localhost/someDB"
+  ignored_databases = ["template0", "template1"]
+```

plugins/inputs/postgresql/postgresql.go

@@ -43,7 +43,7 @@ var sampleConfig = `
   # ignored_databases = ["postgres", "template0", "template1"]
 
   ## A list of databases to pull metrics about. If not specified, metrics for all
-  ## databases are gathered.  Do NOT use with the 'ignore_databases' option.
+  ## databases are gathered.  Do NOT use with the 'ignored_databases' option.
   # databases = ["app_production", "testing"]
 `