HACKING.md: Add code overview section.

HACKING.md

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+## Code overview
+
+The following is a brief code overview / general introduction for those wanting
+to hack on sway.
+
+### wlc
+
+In Wayland the compositor is the display server. That's a design decision that
+brings several advantages, but the downside is that all compositors need to
+implement an entire display server as well.
+
+To aid the situation there are several *wayland display servers* being
+implemented as libraries so that compositors can stick to doing compositing and
+leave the low level details to one of those libraries. In sway that library is
+`wlc`, and it handles tty switching, logind sessions, input, everything that
+deals with the GPU, and just about everything concerning the Wayland protocol
+itself (as of writing there's not a single call to any wayland functions inside
+of sway). sway communicates with wlc via a callback api found in
+`sway/handlers` (`wlc_interface`). The code in that file deals with all the
+entry points from wlc to sway.
+
+### Commands
+
+Being a tiling window manager, controlling it via commands is an important part
+of its functionality, and `sway/commands` which deals with that is by far the
+biggest file in the codebase.
+
+There are multiple ways to trigger a command: via the keyboard, via the config
+file, or via the IPC interface.
+
+### IPC
+
+i3 has an IPC interface (it creates a socket that applications can connect to
+and issue commands or queries via its protocol), and sway replicates that
+protocol (so e.g. `i3-msg` can be used with sway by simply changing the socket,
+e.g. `i3-msg -s $(sway --get-socketpath)`). The code for that lies in
+`sway/ipc`.
+
+### Config
+
+The config state and loading the config file lies in `sway/config`. Since the
+config file is simply a list of commands, that code mostly just parses the text
+and then hands commands off to `commands` for execution.
+
+### Pointer handling
+
+The mouse has buttons, state (due to buttons pressed, e.g. "dragging",
+"resizing" etc.) and movement. Most code related to that lies in
+`sway/input_state`.
+
+### Containers
+
+In traditional *floating* window managers, all windows (or *views* as they're
+called in sway) are placed anywhere on the screen. In a tiling window manager
+like sway the views are *arranged* by the compositor, and the user mostly just
+manipulates the arrangement via commands (floating views are also supported).
+
+In sway, each *output* (a physical monitor) has one or more *workspaces* which
+has one or more *views* (the actual windows). In order to keep track of the
+arrangement of the views, sway organizes everything in a tree of *containers*.
+Each of the previously mentioned things is a type of container. In addition
+there's a type of container called *container* which is needed to arrange other
+containers as siblings (horizontal or vertical layout), and a *root container*
+which exists for practical reasons.
+
+`sway/containers` contains the code for this and understanding containers is
+essential in understanding sway.
+
+Also, the code that actually arranges the different views lays in
+`sway/layout`.
+
+### Focus
+
+When changing workspace, changing output, changing view or just moving the
+pointer you change which view has *focus*. The code for handling this and
+e.g. deciding what view receives input events is handled in `sway/focus`.
+
+### Notes
+
+As sway is a work in progress, as of writing it is still not versioned. Use the
+`master` branch of sway and wlc for now.