update dependencies.md for macOSX (#3899)

add instructions regarding postgresql and bash user/version required for a proper MacOSX install of PeerTube

Co-authored-by: Rozen10 <58482276+Rozen10@users.noreply.github.com>
Co-authored-by: Rigel Kent <par@rigelk.eu>

support/doc/dependencies.md

@@ -281,17 +281,34 @@ service nginx start
 
 1. Add the packages:
 
+```sh
+brew install bash ffmpeg nginx postgresql openssl gcc make redis git yarn
 ```
-brew install ffmpeg nginx postgresql openssl gcc make redis git yarn
+
+You may need to update your default version of bash.
+
+**How to change your default shell**
+
+```sh
+which -a bash # Check where bash is installed
+bash --version # You need a version at least as recent as 4.0
+sudo vim /etc/shells # Add in this file : /usr/local/bin/bash
+chsh -s /usr/local/bin/bash # To set the brew-installed bash as default bash
 ```
 
+In a new shell, type `bash --version` to assert your changes took effect and 
+correctly modified your default bash version.
+
 2. Run the services:
 
-```
+```sh
 brew services run postgresql
 brew services run redis
 ```
 
+On macOS, the `postgresql` user can be `_postgres` instead of `postgres`.
+If `sudo -u postgres createuser -P peertube` gives you an error, you can try `sudo -u _postgres createuser -U peertube`.
+
 ## Gentoo
 
 1. Add this to ``/etc/portage/sets/peertube``: