feat(security): Adds security policy to repo

[darigovresearch]

Closes #370 if merged

Changes:

Adds placeholder Security.md adapted from another repo

Adapted from https://github.com/NethermindEth/nethermind/blob/master/SECURITY.md

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Other (please describe):

Testing

Requires testing

• Yes
• No

In case you checked yes, did you write tests?

• Yes
• No

Comments about testing , should you have some (optional)

[stdevMac]

@darigovresearch thanks so much for this!

Would be possible for you to add the following section on the readme in top of the License one?

## 🛡️ Security 

If you believe you have found a security vulnerability in our code, please report it to us as described in our [security policy](SECURITY.md).

[stdevMac]

With that small change, will be ready to merge, good catch! @darigovresearch