postgres: add README (#7217)

plugins/postgres/README.md

+# Postgres plugin
+
+This plugin adds some aliases for useful Postgres commands.
+
+:warning: this plugin works exclusively with Postgres installed via Homebrew on OSX
+because Postgres paths are hardcoded to `/usr/local/var/postgres`.
+
+To use it, add `postgres` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... postgres)
+```
+
+## Aliases
+
+| Alias       | Command                                                                         | Description                                                 |
+|-------------|---------------------------------------------------------------------------------|-------------------------------------------------------------|
+| startpost   | `pg_ctl -D /usr/local/var/postgres -l /usr/local/var/postgres/server.log start` | Start postgres server                                       |
+| stoppost    | `pg_ctl -D /usr/local/var/postgres stop -s -m fast`                             | Stop postgres server                                        |
+| restartpost | `stoppost && sleep 1 && startpost`                                              | Restart (calls stop, then start)                            |
+| reloadpost  | `pg_ctl reload -D /usr/local/var/postgres -s`                                   | Reload postgres configuration (some setting require restart)|
+| statuspost  | `pg_ctl status -D /usr/local/var/postgres -s`                                   | Check startus of postgres server (running, stopped)         |