Updating M1 installing instructions (#51)

Installing-ZSH.md

@@ -52,7 +52,7 @@ To set zsh as your default shell, execute the following assuming a default insta
   For m1 macs:
 
   ```sh
-  chsh -s /opt/homebrew/bin/zsh
+  chsh -s $(which zsh)
   ```
 
   For intel macs:
@@ -67,6 +67,12 @@ To set zsh as your default shell, execute the following assuming a default insta
   chsh -s /bin/zsh
   ```
 
+If you get an error for non-stadard shell you can try running first 
+
+```sh
+sudo sh -c "echo $(which zsh) >> /etc/shells"
+```
+
 Assuming you have [Homebrew](https://brew.sh/) installed. If not, most versions of
 **macOS** ship zsh by default, but it's normally an older version.  Alternatively, you may
 also use [MacPorts](https://www.macports.org/)