Auto merge of #27960 - servo:readme.ubuntu.20.04, r=jdm

Document python2 special requirements for Ubuntu 20.04+

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because

README.md

@@ -72,6 +72,15 @@ sudo apt install python-virtualenv python-pip
 ./mach bootstrap
 ```
 
+If you are on Ubuntu 20.04 or higher you need to do the following to install the missing python2 bits:
+
+```sh
+curl https://bootstrap.pypa.io/get-pip.py -sSf -o get-pip.py
+python2 get-pip.py
+python2 -m pip install virtualenv
+sudo apt install python-dev
+```
+
 If `./mach bootstrap` doesn't work, file a bug, and, run the commands below:
 
 ``` sh