Add an FAQ document

This will serve as a container for more in-depth documentation to supplement the fundamental reference provided in the
readme.

Author: per1234

README.md

@@ -231,5 +231,6 @@ Set to `true` to cause the action to record the compiler warning count for each
 ## Additional resources
 
 - [Introductory article about **arduino/compile-sketches**](https://blog.arduino.cc/2021/04/09/test-your-arduino-projects-with-github-actions/)
+- [Frequently asked questions about **arduino/compile-sketches**](docs/FAQ.md#frequently-asked-questions)
 - [**GitHub Actions** documentation](https://docs.github.com/actions/learn-github-actions/understanding-github-actions)
 - [Discuss or request assistance on **Arduino Forum**](https://forum.arduino.cc/)

docs/FAQ.md

@@ -0,0 +1,43 @@
+# Frequently Asked Questions
+
+## How can I install dependencies of a boards platform?
+
+### Managed Dependencies
+
+The Arduino **Boards Manager** system installs tool dependencies along with a platform. When you specify a [**Boards Manager**-sourced platform dependency](../README.md#boards-manager) via the action's [`platforms` input](../README.md#platforms) the managed platform dependencies are installed automatically.
+
+If an alternative [platform dependency source](../README.md#supported-platform-sources) is used this automatic tool dependency installation does not occur. The convenient way to install the tool dependencies in this case is to install a **Boards Manager**-sourced platform that has a dependency on the required tools in addition to the target platform from the alternative source.
+
+---
+
+**Example:**
+
+```yaml
+- uses: arduino/compile-sketches@v1
+  with:
+    platforms: |
+      # Use Boards Manager to install the latest release of the platform to get the toolchain.
+      - name: arduino:avr
+      # Overwrite the Boards Manager installation with the development version of the platform.
+      - source-url: https://github.com/arduino/ArduinoCore-avr.git
+        name: arduino:avr
+```
+
+---
+
+### External Dependencies
+
+Arduino boards platforms typically bundle all dependencies. However, there are some platforms that require the user to manually install dependencies on their system in order to use the platform.
+
+The **arduino/compile-sketches** action runs in the same environment as the rest of the steps of the [workflow job](https://docs.github.com/actions/using-jobs/using-jobs-in-a-workflow), which means you can simply perform the dependency installation in a prior [step](https://docs.github.com/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idsteps) of the job.
+
+---
+
+**Example:**
+
+```yaml
+- run: <some dependency installation command>
+- uses: arduino/compile-sketches@v1
+```
+
+---