Update licence file to mention AUTHORS file (release versions only).
Update licence file to mention AUTHORS file (release versions only).

Refers to #3701
kalj/dealii,danshapero/dealii,EGP-CIG-REU/dealii,sairajat/dealii,sairajat/dealii,naliboff/dealii,EGP-CIG-REU/dealii,naliboff/dealii,danshapero/dealii,danshapero/dealii,kalj/dealii,EGP-CIG-REU/dealii,danshapero/dealii,danshapero/dealii,sairajat/dealii,naliboff/dealii,sairajat/dealii,kalj/dealii,kalj/dealii,sairajat/dealii,danshapero/dealii,danshapero/dealii,sairajat/dealii,EGP-CIG-REU/dealii,kalj/dealii,naliboff/dealii,kalj/dealii,sairajat/dealii,EGP-CIG-REU/dealii,naliboff/dealii,naliboff/dealii,EGP-CIG-REU/dealii,kalj/dealii,naliboff/dealii,EGP-CIG-REU/dealii