Create README.md as a mini-overview of PCRE2.

README.md

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+# PCRE2 - Perl-Compatible Regular Expressions
+
+The PCRE2 library is a set of C functions that implement regular expression
+pattern matching using the same syntax and semantics as Perl 5. PCRE2 has its
+own native API, as well as a set of wrapper functions that correspond to the
+POSIX regular expression API. The PCRE2 library is free, even for building 
+proprietary software. It comes in three forms, for processing 8-bit, 16-bit,
+or 32-bit code units, in either literal or UTF encoding.
+
+PCRE2 was first released in 2015 to replace the API in the original PCRE 
+library, which is now obsolete and no longer maintained. As well as a more
+flexible API, the code of PCRE2 has been much improved since the fork.
+ 
+## Download
+
+As well as downloading from this site, you can download PCRE2 or the older,
+unmaintained PCRE1 library from an *unofficial* mirror at SourceForge:
+
+    https://sourceforge.net/projects/pcre/files/
+
+You can check out the PCRE2 source code via Git or Subversion:
+
+    git clone https://github.com/PhilipHazel/pcre2.git
+    svn co    https://github.com/PhilipHazel/pcre2.git
+
+## Contributed Ports
+
+If you just need the command-line PCRE2 tools on Windows, precompiled binary
+versions are available here:
+
+    http://www.rexegg.com/pcregrep-pcretest.html
+
+A PCRE2 port for z/OS, a mainframe operating system which uses EBCDIC as its
+default character encoding, can be found here:
+
+    http://www.cbttape.org/ (File 939)
+
+## Documentation
+
+You can read the PCRE2 documentation here:
+
+https://philiphazel.github.io/pcre2/doc/html/index.html
+
+Comparisons to Perl's regular expression semantics can be found in the
+community authored Wikipedia entry for PCRE.
+
+There is a curated summary of changes for each PCRE release, copies of
+documentation from older releases, and other useful information from the third
+party authored RexEgg PCRE Documentation and Change Log page:
+
+    http://www.rexegg.com/pcre-documentation.html
+
+## Contact
+
+To report a problem with the PCRE2 library, or to make a feature request, please
+use the PCRE2 GitHub issues tracker. There is a mailing list for discussion of
+ PCRE2 issues and development at pcre2-dev@googlegroups.com, which is where any
+announcements will be made. You can browse the list archives here:
+
+    https://groups.google.com/g/pcre2-dev
+