docs: Add valgrind as test dependencies in README.md

[yin19941005]

Please kindly add valgrind as test dependencies in README.md. I am getting error when running the complete test suite without valgrind:

FileNotFoundError: [Errno 2] No such file or directory: 'valgrind'

/usr/lib/python3.8/subprocess.py:1704: FileNotFoundError
----------------------------- Captured stdout call -----------------------------
ID=ubuntu
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
Linux bb-ubuntu-68 5.13.0-51-generic #58~20.04.1-Ubuntu SMP Tue Jun 14 11:29:12 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
. > valgrind -s --error-exitcode=1 --leak-check=full --show-leak-kinds=all /home/user/OpenQuantumSafe/liboqs/build/tests/test_kem Classic-McEliece-6960119f
============= 124 failed, 637 passed, 260 skipped in 39.41 seconds =============
FAILED: tests/CMakeFiles/run_tests
cd /home/user/OpenQuantumSafe/liboqs && /usr/bin/cmake -E env OQS_BUILD_DIR=/home/user/OpenQuantumSafe/liboqs/build python3 -m pytest --verbose --numprocesses=auto --ignore=scripts/copy_from_upstream/repos
ninja: build stopped: subcommand failed.

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the the list of algorithms available -- either adding, removing, or renaming? (If so, PRs in OQS-OpenSSL, OQS-BoringSSL, and OQS-OpenSSH will also be required by the time this is merged.)

[baentsch]

Thank you very much for your contribution. To minimize the amount of text on the page, the valgrind dependency should be added to the other packages required to be installed (earlier on the page), though: Please update your PR accordingly.

[yin19941005]

Ok, sure no problem. Here you go.

[baentsch]

Thanks!