Add developer docs for Vector API

Author: pettyjamesm

.github/DEVELOPMENT.md

@@ -144,6 +144,23 @@ allows static code analysis tools (e.g. Error Prone's `MissingCasesInEnumSwitch`
 check) report a problem when the enum definition is updated but the code using
 it is not.
 
+### Vector API
+It's safe to assume that the JVM has the Vector API
+([JEP 508](https://openjdk.org/jeps/508)) enabled and available at runtime, but
+not safe to assume that the Vector API implementation will perform faster than
+equivalent scalar code on whatever hardware the engine happens to be running on.
+
+When adding implementations that use the Vector API, be sure to:
+* Provide an equivalent scalar implementation in code, if one does not already
+exist.
+* Use configuration flags and hardware support detection to ensure that
+vectorized implementation is only selected when running on hardware where it is
+expected to perform better than its scalar equivalent.
+* Add tests that ensure the behavior of the vectorized and scalar
+implementations match.
+* Include micro-benchmarks that demonstrate the performance benefits of the
+vectorized implementation compared to the scalar equivalent logic.
+
 ## Keep pom.xml clean and sorted
 
 There are several plugins in place to keep pom.xml clean.