Add documentation about Alpine type (#697)

Signed-off-by: John Speed Meyers <jsmeyers@chainguard.dev>

pkg/types/alpine/README.md

+**Alpine Type Data Documentation**
+
+This document notes additional information (beyond that found in the Alpine schema) about the values associated with
+each field such as the format in which the data is stored and any necessary transformations.
+
+Further information about pkginfo fields and the .PKGINFO file can be found on the [Alpine documentation website](https://wiki.alpinelinux.org/wiki/Alpine_package_format#.PKGINFO).
+
+The value of the `publicKey` `content` field ought to be Base64-encoded.
+
+**How do you identify an object as an Alpine object?**
+
+The "Body" field will include an "AlpineModel" field.