Updated the changelog.txt

Uploaded the prebuilt image for IR ACS v2.0.0_BETA-0

Co-authored-by: Amrathesh <amrathesh@arm.com>

Uploaded the pre-built SystemReady ACS SR image.
Updated the changelog.txt with the changes of this release

 1. Support to build IR ACS live image : build scripts and build instructions in documentation
 2. Upgrading ACS image to v5.13 kernel
 3. Refinement of UEFI-SCT Sequence files for IR and ES and verification
 4. Migrate to the version V21.08.00 of FWTS
 5. BBR: Migrate to edk2-test-stable202108 tag of SCT
 6. General improvements in scripts
 7. Bug Fixes

Co-authored-by: Edhaya Chandran <edhaya.chandran@arm.com>
Co-authored-by: Chetan Rathore <chetan.rathore@arm.com>
Co-authored-by: Jiss Jose <jiss.jose@arm.com>

Signed-off-by: jisjos01 <jiss.jose@arm.com>

Signed-off-by: edhay <edhaya.chandran@arm.com>