Add explicit instructions for Zsh in MacOS

Indirect ones with errata have proven to confuse users.
Fixes #1947, #1948

README.md

@@ -263,6 +263,22 @@ easy to fork and contribute any changes back upstream.
 
       - For **Zsh**:
 
+         - **MacOS, if Pyenv is installed with Homebrew:**
+
+            ~~~ zsh
+            echo 'eval "$(pyenv init --path)"' >> ~/.zprofile
+            ~~~
+         
+         - **MacOS, if Pyenv is installed with a Git checkout:**
+         
+            ~~~ zsh
+            echo 'export PYENV_ROOT="$HOME/.pyenv"' >> ~/.zprofile
+            echo 'export PATH="$PYENV_ROOT/bin:$PATH"' >> ~/.zprofile
+            echo 'eval "$(pyenv init --path)"' >> ~/.zprofile
+            ~~~
+
+         - **Other OSes:**
+         
            Same as for Bash above, but add the commands into both `~/.profile`
            and `~/.zprofile`.