[security] add prose explaining OpenSSF CII Best Practices badge results

Fixes openjs-foundation/security-collab-space#35.

.github/SECURITY.md

@@ -1,3 +1,23 @@
 # Security
 
 Please email [@ljharb](https://github.com/ljharb) or see https://tidelift.com/security if you have a potential security vulnerability to report.
+
+## OpenSSF CII Best Practices
+
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/684/badge)](https://bestpractices.coreinfrastructure.org/projects/684)
+
+There are three “tiers”: passing, silver, and gold.
+
+### Passing
+We meet 100% of the “passing” criteria.
+
+### Silver
+We meet 95% of the “silver” criteria. The gaps are as follows:
+  - we do not have a DCO or a CLA process for contributions.
+  - because we only have one maintainer, the project has no way to continue if that maintainer stops being active.
+  - we do not currently document “what the user can and cannot expect in terms of security” for our project. This is planned to be completed in 2023.
+
+### Gold
+We meet 65% of the “gold” criteria. The gaps are as follows:
+  - we do not yet have the “silver” badge; see all the gaps above.
+  - We do not include a copyright or license statement in each source file. Efforts are underway to change this archaic practice into a suggestion instead of a hard requirement.