Add HACKING.md

HACKING.md

@@ -0,0 +1,35 @@
+## Logging
+
+Use `sway_log(importance, fmt, ...)` to log. The following importances are
+available:
+
+* `L_DEBUG`: Debug messages, only shows with `sway -d`
+* `L_INFO`: Informational messages
+* `L_ERROR`: Error messages
+
+`sway_log` is a macro that calls `_sway_log` with the current filename and line
+number, which are written into the log with your message.
+
+## Assertions
+
+In the compositor, assertions *must not* be fatal. All error cases must be
+handled as gracefully as possible - crashing the compositor will make the user
+lose all of their work.
+
+Use `sway_assert(condition, fmt, ...)` to perform an assertion. This returns
+`condition`, which you must handle if false. An error will be logged if the
+assertion fails.
+
+Outside of the compositor (swaymsg, swaybar, etc), using `assert.h` is
+permitted.
+
+## Building against a local wlc
+
+1. Build wlc as described [here](https://github.com/Cloudef/wlc#building)
+2. Inside your sway source folder, tell `cmake` to use your local version of wlc:
+
+```bash
+cmake \
+    -DWLC_LIBRARIES=path/to/wlc/target/src/libwlc.so \
+    -DWLC_INCLUDE_DIRS=path/to/wlc/include .
+```