docs: Add a section on adding new host modules to Code_Overview.md

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Code_Overview.md

@@ -29,6 +29,8 @@ files.
 The **scripts/** directory contains build-time scripts useful for
 compiling the micro-controller code.
 
+The **test/** directory contains automated test cases.
+
 During compilation, the build may create an **out/** directory. This
 contains temporary build time objects. The final micro-controller
 object that is built is **out/klipper.elf.hex** on AVR and
@@ -77,7 +79,7 @@ interrupts disabled.
 Much of the functionality of the micro-controller involves working
 with General-Purpose Input/Output pins (GPIO). In order to abstract
 the low-level architecture specific code from the high-level task
-code, all GPIO events are implemented in architectures specific
+code, all GPIO events are implemented in architecture specific
 wrappers (eg, **src/avr/gpio.c**). The code is compiled with gcc's
 "-flto -fwhole-program" optimization which does an excellent job of
 inlining functions across compilation units, so most of these tiny
@@ -212,6 +214,76 @@ ToolHead and kinematic classes. It's this part of the code which
 specifies the movements and their timings. The remaining parts of the
 processing is mostly just communication and plumbing.
 
+Adding a host module
+====================
+
+The Klippy host code has a dynamic module loading capability. If a
+config section named "[my_module]" is found in the printer config file
+then the software will automatically attempt to load the python module
+klippy/extras/my_module.py . This module system is the preferred
+method for adding new functionality to Klipper.
+
+The easiest way to add a new module is to use an existing module as a
+reference - see **klippy/extras/servo.py** as an example.
+
+The following may also be useful:
+* Execution of the module starts in the module level `load_config()`
+  function (for config sections of the form [my_module]) or in
+  `load_config_prefix()` (for config sections of the form
+  [my_module my_name]). This function is passed a "config" object and
+  it must return a new "printer object" associated with the given
+  config section.
+* During the process of instantiating a new printer object, the config
+  object can be used to read parameters from the given config
+  section. This is done using `config.get()`, `config.getfloat()`,
+  `config.getint()`, etc. methods. Be sure to read all values from the
+  config during the construction of the printer object - if the user
+  specifies a config parameter that is not read during this phase then
+  it will be assumed it is a typo in the config and an error will be
+  raised.
+* Use the `config.get_printer()` method to obtain a reference to the
+  main "printer" class. This "printer" class stores references to all
+  the "printer objects" that have been instantiated. Use the
+  `printer.lookup_object()` method to find references to other printer
+  objects. Almost all functionality (even core kinematic modules) are
+  encapsulated in one of these printer objects. Note, though, that
+  when a new module is instantiated, not all other printer objects
+  will have been instantiated. The "gcode" and "pins" modules will
+  always be available, but for other modules it is a good idea to
+  defer the lookup.
+* Define a `printer_state()` method if the code needs to be called
+  during printer setup and/or shutdown. This method is called twice
+  during setup (with "connect" and then "ready") and may also be
+  called at run-time (with "shutdown" or "disconnect"). It is common
+  to perform "printer object" lookup during the "connect" and "ready"
+  phases.
+* If there is an error in the user's config, be sure to raise it
+  during the `load_config()` or `printer_state("connect")` phases. Use
+  either `raise config.error("my error")` or `raise
+  printer.config_error("my error")` to report the error.
+* Use the "pins" module to configure a pin on a micro-controller. This
+  is typically done with something similar to
+  `printer.lookup_object("pins").setup_pin("pwm",
+  config.get("my_pin"))`. The returned object can then be commanded at
+  run-time.
+* If the module needs access to system timing or external file
+  descriptors then use `printer.get_reactor()` to obtain access to the
+  global "event reactor" class. This reactor class allows one to
+  schedule timers, wait for input on file descriptors, and to "sleep"
+  the host code.
+* Do not use global variables. All state should be stored in the
+  printer object returned from the `load_config()` function. This is
+  important as otherwise the RESTART command may not perform as
+  expected. Also, for similar reasons, if any external files (or
+  sockets) are opened then be sure to close them from the
+  `printer_state("disconnect")` callback.
+* Avoid accessing the internal member variables (or calling methods
+  that start with an underscore) of other printer objects. Observing
+  this convention makes it easier to manage future changes.
+* If submitting the module for inclusion in the main Klipper code, be
+  sure to place a copyright notice at the top of the module. See the
+  existing modules for the preferred format.
+
 Adding new kinematics
 =====================