Update INSTALL.md
Update INSTALL.md

Add `vagrant exec` instructions
hennevogel/osem,openSUSE/osem,bear454/osem,rishabhptr/osem,bear454/osem,SeaGL/osem,BelieveC/osem,differentreality/osem,differentreality/osem,hennevogel/osem,hennevogel/osem,openSUSE/osem,ChrisBr/osem,ChrisBr/osem,kormoc/osem,rishabhptr/osem,BelieveC/osem,rishabhptr/osem,kormoc/osem,namangupta01/osem,namangupta01/osem,differentreality/osem,SeaGL/osem,ChrisBr/osem,ChrisBr/osem,kormoc/osem,Shriyanshagro/osem,BelieveC/osem,differentreality/osem,bear454/osem,kormoc/osem,SeaGL/osem,bear454/osem,Shriyanshagro/osem,Shriyanshagro/osem,rishabhptr/osem,openSUSE/osem,AndrewKvalheim/osem,AndrewKvalheim/osem,hennevogel/osem,BelieveC/osem,Shriyanshagro/osem,AndrewKvalheim/osem,namangupta01/osem,namangupta01/osem