Update README.md

Fix broken repository tool diagram link.
Modify installation setup.

Support the following installation setup:
# Minimal install intended for clients (pure Python, only ed25519 signatures).
pip install tuf

# Optional installation required by the TUF repository tools (RSA keys and rsassa-pss
# signatures, faster ed25519 cryptographic computations, general-purpose cryptography, etc.)
pip install tuf[tools]

Re-implement the fix for issue #167.

Update comments and docstrings related to the fixes above.
Merge remote-tracking branch 'upstream/develop' into develop