Add bug-bounty page

[danfinlay]

We need a highly visible place for security researchers to find our bug bounty program and learn our preferred process of responsible disclosure.

Example of our current biggest bug bounty:
MetaMask/metamask-extension#3127

A couple examples from other projects:

• https://aave.com/bug-bounty
• https://www.argent.xyz/argent-bug-bounty/

[cjeria]

From Whymarrh:

The information about a bug bounty should be in MetaMask/.github#1 which can be updated as needed—the website could just link to that

[jlazoff]

Created already :) https://metamask.io/security/