Add instructions for accessing IDE's advanced settings

Although the Arduino IDE's primary preferences interface provides all required configuration capabilities, advanced
users may wish to fine tune the behavior of the application or temporarily enable additional log output to use for
troubleshooting problems with the IDE.

The IDE provides such settings in a separate interface.

Previously, the existence and access procedure for these settings was undocumented.

Since this is an advanced capability, the documentation is not appropriate for inclusion with the standard user
documentation on arduino.cc. A file here in the Arduino IDE is used instead. This file will serve as a container for all
such user-targeted documentation.

Author: per1234

docs/advanced-usage.md

@@ -0,0 +1,14 @@
+# Advanced usage
+
+## Advanced settings
+
+The Arduino IDE's primary settings are accessible via the **File > Preferences** menu item. These provide all the configuration capability required by the average user to develop sketches.
+
+Arduino IDE has some additional settings which may be of interest to advanced users who want to do things such as fine tune the behavior of the application or increase log output while investigating a problem.
+
+These advanced settings can be accessed by the following procedure:
+
+1. Press the <kbd>**Ctrl**</kbd>+<kbd>**Shift**</kbd>+<kbd>**P**</kbd> keyboard shortcut (<kbd>**Command**</kbd>+<kbd>**Shift**</kbd>+<kbd>**P**</kbd> for macOS users) to open the "**Command Palette**".
+1. Select the "**Preferences: Open Settings (UI)**" command from the menu.
+
+This will open a "**Preferences**" view in the IDE. Once you are finished adjusting settings, it can be closed by clicking the **X** icon on the "**Preferences**" tab.