Clarify module installation command for >=OSX 10.15 (ethereum#1467)

* Clarify module installation command for >=OSX 10.15

* Improve wording

Co-Authored-By: Piper Merriam <[email protected]>

Author: 2 people

docs/README-osx.md

@@ -19,3 +19,14 @@ brew install openssl libffi autoconf automake libtool
 ```sh
 brew install leveldb
 ```
+
+> If you are on `>=OSX 10.15 Catalina` you may encounter the following error with the default `ZSH` shell. This can be fixed by wrapping the `[dev]` part in quotes.
+```sh
+pip install -e .[dev]
+zsh: no matches found: .[dev]
+```
+
+Run install commands as follows:
+```sh
+pip install -e .'[dev]'
+```