Update documentation regarding sketch projects

Author: MatteoPologruto

docs/sketch-project-file.md

@@ -33,6 +33,8 @@ profiles:
       - <LIB_NAME> (<LIB_VERSION>)
 
   ...more profiles here...
+
+default_profile: <DEFAULT_PROFILE_NAME>
 ```
 
 There is an optional `profiles:` section containing all the profiles. Each field in a profile is mandatory (unless noted
@@ -50,6 +52,7 @@ otherwise below). The available fields are:
 - `libraries:` is a section where the required libraries to build the project are defined. This section is optional.
 - `<LIB_VERSION>` is the version required for the library, for example, `1.0.0`.
 - `<USER_NOTES>` is a free text string available to the developer to add comments. This field is optional.
+- `<DEFAULT_PROFILE_NAME>` is the profile used by default (more on that later)
 
 A complete example of a sketch project file may be the following:
 
@@ -95,6 +98,8 @@ profiles:
       - ArduinoIoTCloud (1.0.2)
       - Arduino_ConnectionHandler (0.6.4)
       - TinyDHT sensor library (1.1.0)
+
+default_profile: nanorp
 ```
 
 ### Building a sketch
@@ -116,6 +121,16 @@ not be used in any way. In other words, the build is isolated from the system an
 specified in the profile: this will ensure that the build is portable and reproducible independently from the platforms
 and libraries installed in the system.
 
+### Using a default profile
+
+If a `default_profile` is specified in the `sketch.yaml` then the “classic” compile command:
+
+```
+arduino-cli compile [sketch]
+```
+
+will, instead, trigger a profile-based build using the default profile indicated in the `sketch.yaml`.
+
 ## Default flags for Arduino CLI usage
 
 The sketch project file may be used to set the default value for some command line flags of the Arduino CLI, in