[readme] Update macOS troubleshooting w/ 10.15 zsh default

The default shell for macOS 10.15 is now zsh, by default a .zshrc file
is not created, so switch instruction for touch ~/.zshrc

README.md

@@ -94,7 +94,9 @@ Since OS X 10.9, `/usr/bin/git` has been preset by Xcode command line tools, whi
 
 If you get `nvm: command not found` after running the install script, one of the following might be the reason:
 
-  - Your system may not have a `.bash_profile` file where the command is set up. Create one with `touch ~/.bash_profile` and run the install script again
+  - Since macOS 10.15, the default shell is `zsh` and nvm will look for `.zshrc` to update, none is installed by default. Create one with `touch ~/.zshrc` and run the install script again.
+
+  - If you use bash, the previous default shell, run `touch ~/.bash_profile` to create the necessary profile file if it does not exist.
 
   - You might need to restart your terminal instance or run `. ~/.nvm/nvm.sh`. Restarting your terminal/opening a new tab/window, or running the source command will load the command and the new configuration.