Add prerequisites section, including being able to use python rather than python3

README.md

@@ -8,6 +8,32 @@ Pmodoro Prompt is extremely simplistic, and only has a description field and aut
 
 ## Installation
 
+### Prerequisites
+
+#### Install venv and pip- and python, too!
+
+```
+sudo apt install python3-venv python3-pip
+```
+
+This will also install python3 if it isn't already.
+
+We don't use venv in these instructions but you can if you want to sort of sandbox this project.
+
+Pip is needed.
+
+#### Make Python 3 the default
+
+```
+sudo su
+update-alternatives --install /usr/bin/python python /usr/bin/python3 1
+exit
+```
+
+If you don't do the above, substitute `python3` for `python` in the following.
+
+### Install
+
 ```
 mkdir -p ~/Projects/agaric/python                                                                              
 git clone git@gitlab.com:agaric/python/parse-timelogs-for-upload.git