Update install instructions for Postgres

This apps works with v11, but not with v13. Adding '|| true' in the
Makefile means the 'bootstrap' rule can be run multiple times, even
if the DB already exists.

Makefile

@@ -24,7 +24,7 @@ NOTIFY_CREDENTIALS ?= ~/.notify-credentials
 .PHONY: bootstrap
 bootstrap: generate-version-file
 	pip3 install -r requirements_for_test.txt
-	createdb notification_api
+	createdb notification_api || true
 	. environment.sh && flask db upgrade
 
 .PHONY: run-flask

README.md

@@ -44,9 +44,15 @@ notify-pass credentials/providers/api_keys
 
 ### Postgres
 
-Install [Postgres.app](http://postgresapp.com/). You will need admin on your machine to do this.
+Install [Postgres.app](http://postgresapp.com/).
 
-Choose the version with Additional Releases - you want 9.6. Once you run the app, open the sidebar, remove the default v11 server and create and initialise a v9.6 server.
+Currently the API works with PostgreSQL 11. After installation, open the Postgres app, open the sidebar, and update or replace the default server with a compatible version.
+
+**Note:** you may need to add the following directory to your PATH in order to bootstrap the app.
+
+```
+export PATH=${PATH}:/Applications/Postgres.app/Contents/Versions/11/bin/
+```
 
 ### Redis