Update lists of supported project types in docs

The documentation for the `project-type` input was written at the time of the initial release of the project. At that time Arduino Lint only had full support for sketch and library project types, but the support for boards platforms and package indexes was incomplete. Since that time, full support has been added for boards platforms and package indexes but the action's documentation was not updated accordingly.

Author: per1234

README.md

@@ -11,6 +11,7 @@ projects:
 - Libraries
 - Sketches
 - Boards platforms
+- Boards Manager package indexes
 
 ## Table of contents
 
@@ -78,6 +79,8 @@ Configures which types of projects to check, along with their subprojects.
 
 - `sketch`
 - `library`
+- `platform` - [boards platform](https://arduino.github.io/arduino-cli/latest/platform-specification/)
+- `package-index` - Boards Manager [package index](https://arduino.github.io/arduino-cli/latest/package_index_json-specification/)
 - `all` - Run checks on any type of project that is detected
 
 **Default**: `all`