[Docs] Updated README with Images/Screenshots folder

Updated README.md documentation file with the new folder Images/Screenshots that should contains any Unity images

Also:

ADDED

• A new function fetchPackages() to .github/scripts/functions.sh that list all packages under ./Packages folder

  PS: This is a first version of this script function. Need more implementations, such as: copy and rename files per package, generate distinct [upm] branches for each package, etc...

• Added Custom.gitgnore section to .gitignore file, with project/package specific ignore rules