Add bit of a README.

modules/foxml_file_validation/README.md

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+# FOXML File Validation
+
+## Introduction
+
+In migrations, we provide URIs in the `foxml://` scheme which do not have
+meaningful file extensions, such as:
+
+- `foxml://object/some:pid`; or,
+- `foxml://datastream/some:pid+DSID+DSID.0`
+
+Given these are served from a dedicated endpoint and the usual extension
+validation does not have anything meaningful to work with, this module wraps
+the base `FileExtension` validation such that it is bypassed when the `uri` of
+the underlying file entity has the `foxml://` scheme. File _not_ in the
+`foxml://` scheme should continue to validate extensions unchanged.
+
+## Installation
+
+Enable the (sub)module.
+
+## Configuration
+
+None.