[skip changelog] Document location priority of custom libraries paths (#783)

Custom libraries paths may be specified via arduino-cli compile --libraries.

Author: per1234

docs/sketch-build-process.md

@@ -73,6 +73,7 @@ The folder name contains the include | `AnAwesomeServoForWhatever`
 
 The "location priority" is determined as follows (in order of highest to lowest priority):
 
+1. The library is in a custom libraries path specified via the [`--libraries` option](../commands/arduino-cli_compile/#options) of `arduino-cli compile` (in decreasing order of priority when multiple custom paths are defined)
 1. The library is in the `libraries` subfolder of the IDE's sketchbook or Arduino CLI's user directory
 1. The library is bundled with the board platform/core ([`{runtime.platform.path}/libraries`](platform-specification.md#global-predefined-properties))
 1. The library is bundled with the [referenced](platform-specification.md#referencing-another-core-variant-or-tool) board platform/core