Add SECURITY.md file

[kujtimprenkuSQA]

Description

This PR adds a SECURITY.md file in the root of the repo, the content is the same as the content found in:
https://github.com/near/wallet-selector/security/policy

This is similar to how other companies have done like the example provided in the linked issue:

Here, you can find an example of what general security.md file should look like.

Closes: #981

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Type of change.

• FIX - a PR of this type patches a bug.
• FEATURE - a PR of this type introduces a new feature.
• BUILD - a PR of this type introduces build changes.
• CI - a PR of this type introduces CI changes.
• DOCS - a PR of this type introduces DOCS improvement.
• STYLE - a PR of this type introduces style changes.
• REFACTOR - a PR of this type introduces refactoring.
• PERFORMANCE - a PR of this type introduces performance changes.
• TEST - a PR of this type adds more tests.
• CHORE - a PR introduces other changes than the specified above.