Add info on the -perl tag variants to the nginx README

nginx/variant-perl.md

@@ -0,0 +1,3 @@
+## `%%IMAGE%%:<version>-perl` / `%%IMAGE%%:<version>-alpine-perl`
+
+Starting with %%IMAGE%%:1.13.0 / mainline and %%IMAGE%%:1.12.0 / stable, the perl module has been removed from the default images. A separate `-perl` tag variant is available if you wish to use the perl module.