Add brief description of wx-specific changes in this repository

Explain how to update to a newer PCRE version.

README-WX.md

@@ -0,0 +1,20 @@
+PCRE sources used by wxWidgets
+==============================
+
+This directory contains PCRE version used by wxWidgets.
+
+It was created by cloning https://github.com/rurban/pcre which is, in turn, a
+Git clone of the official PCRE2 subversion repository.
+
+Updating PCRE
+-------------
+
+Fast-forward master to the new PCRE version, then merge it into wx branch
+using `git merge --no-commit`. Before committing the merge, additionally:
+
+1. Rerun autogen.sh.
+1. Copy `src/pcre2_chartables.c.dist` to `src/pcre2_chartables.c`.
+1. Examine changes to `src/pcre2.h.generic` and `src/config.h.generic` and
+   propagate them to `src/wx/pcre2.h` and `src/wx/config.h` respectively.
+
+Add all changed files to the commit.