Update documentations

[shchae7]

Documentations, including README and comments, are either not up to date or missing in relation to the Maude rules and files in the repository. Below are some examples that require an update.

• README

  • formal-model is now available in a separate repository.

    
    This artifact consists of the following files and directories:
    ...

• formal-model : the source codes of our TEE API formal model, ...

• analysis/threats/msg.maude

  • lack of comments for each operator and rule in file

    op exploitedBefore : Oid Oid Set{Log} -> Bool .
    ...
    op learnBefore : Oid Oid Set{AttackTarget} -> Bool .
    ...  
    crl [learn-attack-targets] :
    ...