Add preferences documentation

Author: topolarity

docs/src/index.md

@@ -12,3 +12,9 @@ Documentation for [HostCPUFeatures](https://github.com/JuliaSIMD/HostCPUFeatures
 ```@autodocs
 Modules = [HostCPUFeatures]
 ```
+
+## Supported Preferences
+
+  - `cpu_target`: if provided, use this string as your CPU target for feature detection instead of `JULIA_CPU_TARGET`
+  - `freeze_cpu_target`: if `true`, "freeze" the features detected based on your precompile-time CPU target and do not perform runtime feature detection
+  - `allow_runtime_invalidation`: if `false`, warn when performing runtime feature detection (instead of invalidating) when CPU features don't match precompile-time