readme: Update README.md file

Update the README.md file to describe the platform fault
detection interface project, including libpfdi,
pfdi-sample-app, pfdi-cli and pfdi-tool.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>
Signed-off-by: Divin Raj <divin.raj@arm.com>