Update contributing docs

[mapelpapel]

Description

When trying to run pytest on Windows, it fails most tests. Somehow, it can't load all modules.

How To Reproduce

Follow Contributing Guide

• clone Scancode
• .\configure (PowerShell, embedded in PyCharm) / configure (cmd)
• .\Scripts\activate (PowerShell) Scripts\activate (cmd)
• pytest [-vvvs -n8]

Log Output

Sadly, a part of the output is cut off (at the end of the log). It's always the same error with different modules.
PowerShell output: https://drive.google.com/file/d/1t-UMEk9nW7K-3NtQRkNhs8xSClMH4PAF/view?usp=sharing
cmd output: https://drive.google.com/file/d/1Ic3dT0aUS1L43Elpjt_hCxwabjwbMKPl/view?usp=sharing

System configuration

• Windows 10 20H2
• Scancode 21.8.4
• Installed from Source (PyCharm Chrome extension / GitBash for Windows)
• PyCharm 2021.2

[mapelpapel]

Ok, I found the Problem myself...
I needed to run configure --dev, which I found in the documentation (but not contributing file)

[AyanSinhaMahapatra]

Reopening this.

The contributing file should be updated (and maybe we should just include the contributing file in docs, instead of having two seperate contributing docs).