Update the CONTRIBUTING notes for developers.

CONTRIBUTING.rst

@@ -13,11 +13,11 @@ For developers
 --------------
 
 The build specifies C99 conformance, all compiler warnings turned on,
-and all warnings considered errors (compiler options ``-std=c99
--Werror -Wall -Wextra``).
+and all warnings considered errors (compiler options
+``-std=c99 -Werror -Wall -Wextra``).
 
 The VMOD has been tested with both the gcc and clang compilers, and
-should always compile and test successfully with both of them.
+MUST always compile and test successfully with both of them.
 
 By default, ``CFLAGS`` is set to ``-g -O2``, so that symbols are
 included in the shared library, and optimization is at level
@@ -28,3 +28,31 @@ For development/debugging cycles, the ``configure`` option
 ``--enable-debugging`` is recommended (off by default). This will turn
 off optimizations and function inlining, so that a debugger will step
 through the code as expected.
+
+The configure script runs a check to determine if the PCRE2 lib
+against which it is built has Unicode enabled. If so, then some vtc
+tests in ``src/tests`` with the suffix ``.yes`` are enabled, by
+temporarily copying them to files with the ``.vtc`` ending, so that
+``make check`` executes them. If not, then tests with the ``.no``
+suffix are enabled. The VMOD MUST always pass all tests under both of
+these conditions; so it is advisable to have the PCRE2 source
+repository on your system, so that you can build it with and without
+Unicode, and test the VMOD against both versions.
+
+Following the PCRE2 test code, the configure script uses ``locale -a``
+to determine if the ``fr_FR`` locale is installed. If so, it runs
+tests using that locale, again by temporarily copying a vtc script
+with the ``.yes`` suffix, or else the ``.no`` suffix. The VMOD MUST
+always pass these tests as well, so it is advisable to enable
+``fr_FR`` on your system (you may need to run a command like
+``locale-gen``).
+
+The VMOD makes no attempt to support some of the more obscure
+properties of PCRE2 that are not likely to be relevant on any platform
+on which Varnish runs. For example, we won't bother with EBCDIC. But
+in general, the VMOD MUST NOT be developed in such a way that it
+assumes or depends on the availability or unavailability of PCRE2's
+build-time features. For example, it MUST be possible to build the
+VMOD against a version of the library for which Unicode and JIT are
+disabled, and then deploy the compiled code in an environment in which
+are those features are available and can be used by the VMOD.