chore: update README.md, add how to install (brianc#3339)

Fix brianc#3314

Author: DavideViolante

README.md

@@ -18,6 +18,11 @@ This repo is a monorepo which contains the core [pg](https://github.com/brianc/n
 - [pg-connection-string](https://github.com/brianc/node-postgres/tree/master/packages/pg-connection-string)
 - [pg-protocol](https://github.com/brianc/node-postgres/tree/master/packages/pg-protocol)
 
+## Install
+```
+npm install pg
+```
+
 ## Documentation
 
 Each package in this repo should have its own readme more focused on how to develop/contribute. For overall documentation on the project and the related modules managed by this repo please see: