[freetype2] gsoc-anurag-docs-2023 64b2b80bc 2/5: [docs] Add documentation on adding new modules to FreeType

[Werner Lemberg]

branch: gsoc-anurag-docs-2023
commit 64b2b80bcc863f47942f230a0b94431324416d14
Author: Anurag Thakur <anurag105csec21@bpitindia.edu.in>
Commit: Anurag Thakur <anurag105csec21@bpitindia.edu.in>

    [docs] Add documentation on adding new modules to FreeType
---
 docs/.gitignore   |  1 +
 docs/MODIFYING.md | 42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/docs/.gitignore b/docs/.gitignore
index d7777e70c..85e163820 100644
--- a/docs/.gitignore
+++ b/docs/.gitignore
@@ -12,6 +12,7 @@ reference/
 !INSTALL_MAC.md
 !INSTALL_UNIX.md
 !INSTALL_VMS.md
+!MODIFYING.md
 
 # MkDocs Config file
 mkdocs.yml
diff --git a/docs/MODIFYING.md b/docs/MODIFYING.md
new file mode 100644
index 000000000..22ef38997
--- /dev/null
+++ b/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/`