howto-setup-build-env.md: Add further instructions

[DhruvaG2000]

It would help if the document mentions the required hardware specs, given here.
Also, while following the instructions on WSL Ubuntu 18.04, The Install Repo step failed for me as the gpg --keyserver keyserver.ubuntu.com --recv-key 8BB9AD793E8E6153AF0F9A4416530D5E920F5C65 is just stuck.

[DhruvaG2000]

Update 1:
There seems to be an issue with keyserver.ubuntu.com and is not accessible by many users currently.

Upon trying another network, I get the following errors:

$ gpg --keyserver keyserver.ubuntu.com --recv-key 8BB9AD793E8E6153AF0F9A4416530D5E920F5C65
gpg: key 16530D5E920F5C65: 4 signatures not checked due to missing keys
gpg: key 16530D5E920F5C65: "Repo Maintainer <repo@android.kernel.org>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1

[realShengYao]

Hi! I tested this command again with my ubuntu 20.04 virtual machine. I didn't get any errors, though. I'm located in Ohio, USA while my Internet provider is Spectrum.