[docs] Add documentation on adding new modules to FreeType

docs/.gitignore

@@ -12,6 +12,7 @@ reference/
 !INSTALL_MAC.md
 !INSTALL_UNIX.md
 !INSTALL_VMS.md
+!MODIFYING.md
 
 # MkDocs Config file
 mkdocs.yml

docs/MODIFYING.md

@@ -0,0 +1,42 @@
+# Modifying FreeType
+
+FreeType follows a modular architecture, i.e. all the features are
+implemented as separate modules. There are separate modules for 
+rasterizers, font parsers, hinting etc. located under the `src/` directory.
+
+(See https://freetype.org/freetype2/docs/design/design-5.html)
+
+To add new features you have to either modify the existing modules or
+add a new module to FreeType.
+
+## Adding a new module to FreeType
+
+Suppose we want to add a new module "example" to FreeType:
+
+1. Create a directory under `src/` having the same name as the module.
+   i.e. `src/example/`
+
+2. Add source files under src/example having `#define FT_MAKE_OPTION_SINGLE_OBJECT`
+   which includes the other files to create the module.
+
+   (See `src/sdf/sdf.c` for reference)
+
+3. Add the module to `include/freetype/config/ftmodule.h` according to whether
+   it is a renderer, font driver or another module like:
+
+    ```C
+    FT_USE_MODULE( FT_Renderer_Class, ft_example_renderer_class )
+    ```
+
+4. Add the module to `modules.cfg` in the toplevel directory:
+    ```
+    RASTER_MODULES += example
+    ```
+
+5. Under `src/example` add `module.mk` and `rules.mk` files to enable compilation
+   with `make`. (See `src/smooth/rules.mk` for reference).
+
+6. Now you can simply compile by using `make` in the toplevel directory and the module
+   should compile.
+
+   You can check out pre-existing modules for reference under `src/`