Small prerequsite for Ubuntu 20.04

[StianOby]

After much trial and error I figured out that one deb package must be installed on Ubuntu 20.04 LTS for build.sh to run properly:

sudo apt install python-yaml

Without this package installed, the yaml file will not be generated automatically, but the first step of the script will fail. If there is another yaml file already in the folder (e.g. because you have tried editing manually) the script will nevertheless continue, thus "masking" the error. Deleting that docker-compose.yaml file before running the script again will make the script stop on the "No module named yaml" error.

[shayaantx]

if you want you can put up a PR updating the readme and/or update the shell script making sure required python binaries are installed (and error if they are not)

not sure if I'll have time this weekend, if I do - I'll do above - unless you wanna do it.

[StianOby]

Added a note in the README (#3), as I was unsure how to best add it to the build script.

[shayaantx]

We can do something with pydoc and grep the modules for ones we care about, or we can use pip list (something like that to make sure required modules are installed)