[config] Flesh out CONFIG.md

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

CONFIG.md

@@ -44,8 +44,89 @@ 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).
+expense of removing functionality (that may not be needed).  The remaining
+options are all enabled by defining pre-processor macros, which can be done
+via `CXXFLAGS` or `CPPFLAGS` similarly.
 
-## Custom configuration
 
-TODO
+## Unicode-functions
+
+Access to Unicode data can be configured at compile time as well as run-time.
+By default, HarfBuzz ships with its own compact subset of properties from
+Unicode Character Database that it needs.  This is a highly-optimized
+implementation that depending on compile settings (optimize-size or not)
+takes around ~40kb or ~60kb.  Using this implementation (default) is highly
+recommended, as HarfBuzz always ships with data from latest version of Unicode.
+This implementation can be disabled by defining `HB_NO_UCD`.
+
+For example, if you are enabling ICU as a built-in option, or GLib, those
+can provide Unicode data as well, so defining `HB_NO_UCD` might save you
+space without reducing functionality (to the extent that the Unicode version
+of those implementations is recent.)
+
+If, however, you provide your own Unicode data to HarfBuzz at run-time by
+calling `hb_buffer_set_unicode_funcs` on every buffer you create, and you do
+not rely on `hb_unicode_funcs_get_default()` results, you can disable the
+internal implementation by defining both `HB_NO_UCD` and `HB_NO_UNICODE_FUNCS`.
+The latter is needed to guard against accidentally building a library without
+any default Unicode implementations.
+
+
+## Font-functions
+
+Access to certain font functionalities can also be configured at run-time.  By
+default, HarfBuzz uses an efficient internal implementation of OpenType
+functionality for this.  This internal implementation is called `hb-ot-font`.
+All newly-created `hb_font_t` objects by default use `hb-ot-font`.  Using this
+is highly recommended, and is what fonts use by default when they are created.
+
+Most embedded uses will probably use HarfBuzz with FreeType using `hb-ft.h`.
+In that case, or if you otherwise provide those functions by calling
+`hb_font_set_funcs()` on every font you create, you can disable `hb-ot-font`
+without loss of functionality by defining `HB_NO_OT_FONT`.
+
+
+## Thread-safety
+
+By default HarfBuzz builds as a thread-safe library.  The exception is that
+the `HB_TINY` predefined configuring (more below) disables thread-safety.
+
+If you do /not/ need thread-safety in the library (eg. you always call into
+HarfBuzz from the same thread), you can disable thread-safety by defining
+`HB_NO_MT`.  As noted already, this is enabled by `HB_TINY`.
+
+
+## Pre-defined configurations
+
+The [`hb-config.hh`](src/hb-config.hh) internal header supports three
+pre-defined configurations as well grouping of various configuration options.
+The pre-defined configurations are:
+
+  * `HB_MINI`: Disables shaping of AAT as well as legacy fonts.  Ie. it produces
+    a capable OpenType shaper only.
+
+  * `HB_LEAN`: Disables various non-shaping functionality in the library.  See
+    the definition for details.
+
+  * `HB_TINY`: Enables both `HB_MINI` and `HB_LEAN` configurations, as well as
+    disabling thread-safety and debugging, and use size-optimized data tables.
+
+
+## Tailoring configuration
+
+Most of the time, one of the pre-defined configuration is exactly what one needs.
+Sometimes, however, the pre-defined configuration cuts out features that might
+be desired in the library.  Unfortunately there is no quick way to undo those
+configurations from the command-line.  But one can add a header file called
+`config-override.h` to undefine certain `HB_NO_*` symbols as desired.  Then
+define `HAVE_CONFIG_OVERRIDE_H` to make `hb-config.hh` include your configuration
+overrides at the end.
+
+
+## Notes
+
+Note that the config option `HB_NO_CFF`, which is enabled by `HB_LEAN` and
+`HB_TINY` does /not/ mean that the resulting library won't work with CFF fonts.
+The library can shape valid CFF fonts just fine, with or without this option.
+This option disables (among other things) the code to calculate glyph exntents
+for CFF fonts.