Merge branch '21.0-directIO-art-adding-README' into '21.0'

Adding README to DirectIOART package & updating generateARTtests.py

See merge request atlas/athena!13589

(cherry picked from commit 66d10442 [formerly 09cf2e2eb31749106e3dbf50c34fd9b7de8eb8f0])

02319117 Added README.md and updated generateARTtests.py
19d20592 Updated info about software releases in which the TFile::Open testa are executed
e6b5db43 Added master/Athena to RecoTF tests
446e423d Again updating README.md...

Former-commit-id: 079087b54c44d72b8099ecd497f92c40fd0322c9