Updated docs

Author: cmaglie

docs/sketch-project-file.md

@@ -34,6 +34,7 @@ profiles:
         platform_index_url: <3RD_PARTY_PLATFORM_DEPENDENCY_URL>
     libraries:
       - <INDEX_LIB_NAME> (<INDEX_LIB_VERSION>)
+      - dependency: <INDEX_LIB_NAME> (<INDEX_LIB_VERSION>)
       - dir: <LOCAL_LIB_PATH>
     port: <PORT_NAME>
     port_config:
@@ -56,8 +57,14 @@ otherwise below). The available fields are:
   information as `<PLATFORM>`, `<PLATFORM_VERSION>`, and `<3RD_PARTY_PLATFORM_URL>` respectively but for the core
   platform dependency of the main core platform. These fields are optional.
 - `libraries:` is a section where the required libraries to build the project are defined. This section is optional.
-  - `<INDEX_LIB_NAME> (<INDEX_LIB_VERSION>)` represents a library from the Arduino Libraries Index, for example,
-    `MyLib (1.0.0)`.
+  - `<INDEX_LIB_NAME> (<INDEX_LIB_VERSION>)` represents a library from the Arduino Libraries Index that is a direct
+    dependency of the sketch, for example, `MyLib (1.0.0)`.
+  - `dependency: <INDEX_LIB_NAME> (<INDEX_LIB_VERSION>)` represents a library from the Arduino Libraries Index that is
+    an "indirect dependency", i.e., a library that is not used directly by the sketch, but is required by other
+    libraries used by the sketch. For example, if `LibUsedInSketch` requires `UtilLib`, this information can be recorded
+    in the profile with the declaration `dependency: UtilLib (1.3.4)`. During sketch compilation, a dependency library
+    is treated the same as a non-dependency library; the only difference is in automatic library management, where a
+    dependency library can be installed or removed together with the library that uses it.
   - `dir: <LOCAL_LIB_PATH>` represents a library installed in the filesystem and `<LOCAL_LIB_PATH>` is the path to the
     library. The path could be absolute or relative to the sketch folder. This option is available since Arduino CLI
     1.3.0.