Prepared the ReleaseNotes.md and cta.spec.in file for the v3.1-6 release

ReleaseNotes.md

-# v3.1-6 NEXT RELEASE
+# v3.1-6
 
 ## Summary
 

cta.spec.in

@@ -488,6 +488,18 @@ Currently contains a helper for the client-ar script, which should be installed
 %attr(0755,root,root) /usr/bin/cta-client-ar-abortPrepare
 
 %changelog
+* Mon Aug 21 2020 julien.leduc (at) cern.ch - 3.1-6
+- cta/CTA#881 cta-fst-gcd logs can be now sent to stdout by command line option for container based deployments
+- cta/CTA#885 cta-admin should be able to query by sys.archive.file_id
+- Upstream eos 4.8.15-1
+- Upstream xrootd 4.12.4-1
+- cta/CTA#890 CTA RPMs should only use the xrootd-client-libs package
+- buildtree installation scripts are made compatible with Centos 7
+- cta/CTA#892 Modified the log level of the triggering of Archive and Retrieve mounts
+- cta/CTA#889 It is not possible to retrieve a file that is not active anymore
+- cta/CTA#877 ObjectStore.RetrieveQueueAlgorithms unit tests fails or succeeds base on version of cmake
+- cta/CTA#888 Garbage collector race condition
+- cta/CTA#891 Corrected Repack Archive subrequest creation time
 * Thu Aug 20 2020 julien.leduc (at) cern.ch - 3.1-5
 - cta/CTA#863 Prevent SQLite database files from being used as the CTA catalogue database backend
 - cta/CTA#861 cta-admin comment column is flush left