Add README for hsregex module

hsregex/README.md

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+<!--
+SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+
+SPDX-License-Identifier: Apache-2.0
+-->
+
+# Evaluate Regular Expressions in OpenPGP Signatures using TCL-Regex
+
+RFC4880 specifies contains a section about RegularExpression subpackets on signatures.
+Within this section, the syntax of the RegularExpression subpackets is defined to be the same as Henry Spencer's "almost public domain" regular expression package.
+
+Since Java's `java.util.regex` syntax is too powerful, this module exists to implement regex evaluation using [tcl-regex](https://github.com/basis-technology-corp/tcl-regex-java)
+which appears to be a Java port of Henry Spencers regex package.
+
+To make use of this implementation, simply call
+```java
+RegexInterpreterFactory.setInstance(new HSRegexInterpreterFactory());
+```