Start to prepare for 2021.05 release: update documentation

Changelog

@@ -2,7 +2,8 @@ Changelog for next release
 
 - not yet decided
 
-Upcoming changes for ELPA 2021.05.001
+Changelog for ELPA 2021.05.001.rc
+
 - allow the user to set the mapping of MPI tasks to GPU id per set/get
 - experimental feature: port to AMD GPUS, works correctly, performance yet
   unclear; only tested --with-mpi=0

RELEASE_NOTES

-This file contains the release notes for the ELPA 2020.11.001 version
+This file contains the release notes for the ELPA 2021.05.001.rc1 version
 
 What is new?
 -------------
-For detailed information about changes since release ELPA 2020.05.001 please have a look at the Changelog file
+For detailed information about changes since release ELPA 2020.11.001 please have a look at the Changelog file
 
 - bugfixes
-- experimental feature: support for ARM SVE
-- experimental feature: complex kernels for arch64 NEON
+- experimental feature: support for AMD GPUs
 
 
 ABI change