Add CONFIG.md

https://github.com/harfbuzz/harfbuzz/issues/1652

CONFIG.md

+# Configuring HarfBuzz
+
+Most of the time you will not need any custom configuration.  The configuration
+options provided by `configure` or `cmake` should be enough.  In particular,
+if you just want HarfBuzz library plus hb-shape / hb-view utilities, make sure
+FreeType and Cairo are available and found during configuration.
+
+If you are building for distribution, you should more carefully consider whether
+you need Glib, ICU, Graphite2, as well as CoreText / Uniscribe / DWrite.  Make
+sure the relevant ones are enabled.
+
+If you are building for custom environment (embedded, downloadable app, etc)
+where you mostly just want to call `hb_shape()` and the binary size of the
+resulting library is very important to you, the rest of this file guides you
+through your options to disable features you may not need, in exchange for
+binary size savings.
+
+## Compiler Options
+
+Make sure you build with your compiler's "optimize for size" option.  On `gcc`
+this is `-Os`, and can be enabled by passing `CXXFLAGS=-Os` either to `configure`
+(sticky) or to `make` (non-sticky).  On clang there is an even more extreme flag,
+`-Oz`.
+
+HarfBuzz heavily uses inline functions and the optimize-size flag can make the
+library smaller by 20% or more.  Moreover, sometimes, based on the target CPU,
+the optimize-size builds perform *faster* as well, thanks to lower code
+footprint and caching effects.  So, definitely try that even if size is not
+extremely tight but you have a huge application.  For example, Chrome does
+that.  Note that this configuration also automatically enables certain internal
+optimizations.  Search for `HB_OPTIMIZE_SIZE` for details, if you are using
+other compilers, or continue reading.
+
+Another compiler option to consider is "link-time optimization", also known as
+'lto'.  To enable that, with `gcc` or `clang`, add `-flto` to both `CXXFLAGS`
+and `LDFLAGS`, either on `configure` invocation (sticky) or on `make` (non-sticky).
+This, also, can have a huge impact on the final size, 20% or more.
+
+Finally, if you are making a static library build or otherwise linking the
+library into your app, make sure your linker removes unused functions.  This
+can be tricky and differ from environment to environment, but you definitely
+want to make sure this happens.  Otherwise, every unused public function will
+be adding unneeded bytes to your binary.
+
+Combining the above three build options should already shrink your library a lot.
+The rest of this file shows you ways to shrink the library even further at the
+expense of removing functionality (that may not be needed).
+
+## Custom configuration
+
+TODO

Makefile.am

@@ -13,6 +13,7 @@ EXTRA_DIST = \
 	README.mingw.md \
 	README.python.md \
 	BUILD.md \
+	CONFIG.md \
 	RELEASING.md \
 	TESTING.md \
 	CMakeLists.txt \

README.md

@@ -17,6 +17,8 @@ For license information, see [COPYING](COPYING).
 
 For build information, see [BUILD.md](BUILD.md).
 
+For custom configurations, see [CONFIG.md](CONFIG.md).
+
 For test execution, see [TESTING.md](TESTING.md).
 
 Documentation: https://harfbuzz.github.io