Update README.md

Author: davidnewhall

README.md

@@ -10,7 +10,7 @@
 brew install python3
 python3 -m venv .venv
 source .venv/bin/activate
-pip3 install mkdocs mkdocs-material mkdocs-material-extensions mkdocs-glightbox
+python3 -m pip install -r requirements.txt
 mkdocs serve
 ```
 
@@ -24,6 +24,6 @@ This section needs filling out.
 sudo apt install python3 || sudo yum install python3
 python3 -m venv .venv
 source .venv/bin/activate
-pip3 install mkdocs mkdocs-material mkdocs-material-extensions mkdocs-glightbox
+python3 -m pip install -r requirements.txt
 mkdocs serve
 ```