Add Hardware Security Module to accounts documentation

[miohtama]

Explain HSM, add an example how to use

What was wrong?

There was no information how to use hardware security module keys with Web3.py.

How was it fixed?

Added documentation bit to accounts, refer to Google Cloud HSM example.

Todo:

• Clean up commit history
• Add or update documentation related to these changes
• Add entry to the release notes

Cute Animal Picture

                                       /;    ;\
                                   __  \\____//
                                  /{_\_/   `'\____
                                  \___   (o)  (o  }
       _____________________________/          :--'
   ,-,'`@@@@@@@@       @@@@@@         \_    `__\
  ;:(  @@@@@@@@@        @@@             \___(o'o)
  :: )  @@@@          @@@@@@        ,'@@(  `===='
  :: : @@@@@:          @@@@         `@@@:
  :: \  @@@@@:       @@@@@@@)    (  '@@@'
  ;; /\      /`,    @@@@@@@@@\   :@@@@@)
  ::/  )    {_----------------:  :~`,~~;
 ;;'`; :   )                  :  / `; ;   
;;;; : :   ;                  :  ;  ; :
`'`' / :  :                   :  :  : :
    )_ \__;      ";"          :_ ;  \_\       `,','
    :__\  \    * `,'*         \  \  :  \   *  8`;'*  *
        `^'     \ :/           `^'  `-^-'   \v/ :  \/
----------  Targon (Ed Wisniewski)