Repository Check: SECURITY.md

[andygrunwald]

What would you like to be added:

A new repository check that checks the existence of an SECURITY.md.

Why is this needed:

• This might be a good place for mentioning how to report a security issue
• It is mentioned to new people who aim to open a new ticket for the first time, see

Anything else we need to know?:

• GitHub-Docs: Adding a security policy to your repository
• facebook/react-native-example