Document Ubuntu VM install issues 

1) 
```
Collecting pyyaml==5.4.1 (from iris==0.0.0)
  Downloading PyYAML-5.4.1.tar.gz (175 kB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 175.1/175.1 kB 26.3 MB/s eta 0:00:00
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  error: subprocess-exited-with-error
```

Solution: `pyyaml>=5.4.1`

That's it!