Document modules (#49)

* Document modules

* Updates

Author: L1ghtmann

_data/docs_sidebar.yaml

@@ -35,6 +35,8 @@
       link: /docs/dm.pl.html
     - title: Swift
       link: /docs/Swift.html
+    - title: Modules
+      link: /docs/Modules.html
 
 - title: Tweak Development
   list:

docs/Modules.md

@@ -0,0 +1,39 @@
+---
+title: Modules
+layout: docs
+---
+
+## Overview
+With Theos being Makefile-based, additional functionality can be provided as-desired through "modules."
+
+Modules are directories containing Makefiles with additional rules, variables, and/or checks that you would like your Theos install to contain. A module structure may look something like
+```
+my-module/
+├── instance
+│   ├── framework.mk
+│   ├── library.mk
+│   └── rules.mk
+├── package
+│   └── deb.mk
+└── package.mk
+```
+where each `.mk` file contains custom code such as
+```makefile
+_THEOS_INTERNAL_LDFLAGS += -rpath my/path/here/ -rpath $(THEOS_PACKAGE_INSTALL_PREFIX)/Library/Frameworks -rpath $(THEOS_PACKAGE_INSTALL_PREFIX)/usr/lib
+```
+
+## Structure
+Modules' structure and the respective `.mk` filenames tell Theos where to add your provided functionality. For example, `my-module/package/deb.mk` tells Theos to add the functionality contained in the module's `deb.mk` file to `$(THEOS_MAKE_PATH)/package/deb.mk`. Providing additional functionality through modules is only supported for specific parts of Theos, however. These parts include:
+
+- `rules.mk`
+- `common.mk`
+- `package.mk`
+- `platform/*.mk`
+- `targets/**/*.mk`
+- `package/*.mk`
+- `install/*.mk`
+- `instance/*.mk`
+  - Except `aggregate.mk`
+- `master/*.mk`
+
+Modules should be placed in `$(THEOS_MODULE_PATH)` and are imported by Theos *only* when you explicitly ask them to be through `MODULES = my-module`. The exception to this are package schemes, which can be provided in module form (see `$THEOS/vendor/mod/rootless` for an example). For package schemes, the scheme specified by `THEOS_PACKAGE_SCHEME` will be imported without needing to specify the module explicitly via `MODULES`. These are "enabled" through Makefile's `-include` feature and are typically included *after* all of the stock Theos functionality has been defined. Thus, modules support all Theos variables that have been defined by the time your specific Module is imported.