docs(readme): state pg@7 is not supported yet (#8523)

README.md

@@ -27,10 +27,10 @@ Sequelize is a promise-based Node.js ORM for Postgres, MySQL, SQLite and Microso
 $ npm install --save sequelize
 
 # And one of the following:
-$ npm install --save pg pg-hstore
+$ npm install --save pg@6 pg-hstore # Note that `pg@7` is not supported yet
 $ npm install --save mysql2
 $ npm install --save sqlite3
-$ npm install --save tedious // MSSQL
+$ npm install --save tedious # MSSQL
 ```
 
 Sequelize follows [SEMVER](http://semver.org). Supports Node v4 and above to use ES6 features.