Create OpenBMC GitHub security policy

[Joseph Reynolds]

Can we tell GitHub the OpenBMC security policy at 
https://github.com/openbmc/openbmc/security (access via the "security" 
tab shown on https://github.com/openbmc/openbmc)?

GitHub suggests creating a file called openbmc/SECURITY.md.  I've used 
the format provided and customized the content for the OpenBMC project.

Thoughts?  Should I push a review for this?

- Joseph

```
# Security Policy

## Supported Versions

Security updates are generally provided only on the development branch.
See the [release 
notes](https://github.com/openbmc/docs/blob/master/release/release-notes.md).

## Reporting a Vulnerability

See [how to report a security 
vulnerability](https://github.com/openbmc/docs/blob/master/security/how-to-report-a-security-vulnerability.md).
```