Add documentation for the "skip-packages" option

Resolve #217

Author: valeros

core/userguide/cmd_check.rst

@@ -110,6 +110,18 @@ a tool selected for performing check. Possible values described in
 
 Example: ``pio check --fail-on-defect=low --fail-on-defect=medium``
 
+.. option::
+    --skip-packages
+
+Exclude underlying third-party packages from the checking process. By default, PlatformIO
+passes frameworks and toolchains include paths required by internal analysis tools to
+properly analyze project sources. Some of the supported analysis tools use their own
+preprocessor which may fail to parse perfectly valid code and thus provide empty or
+partial check reports. This option is useful when developers have no control over this
+third-party code and want to perform analysis at least on project sources.
+
+Example: ``pio check --skip-packages``
+
 .. option::
     -s, --silent
 

projectconf/section_env_check.rst

@@ -117,3 +117,31 @@ Another option for filtering source files is :option:`pio check --severity` comm
     platform = ...
     board = ...
     check_severity = medium, high
+
+
+.. _projectconf_check_skip_packages:
+
+``check_skip_packages``
+^^^^^^^^^^^^^^^^^^^^^^^
+
+Type: ``Bool (yes or no)`` | Multiple: ``No`` | Default: ``no``
+
+Exclude underlying third-party packages from the checking process. By default, PlatformIO
+passes frameworks and toolchains include paths required by internal analysis tools to
+properly analyze project sources. Some of the supported analysis tools use their own
+preprocessor which may fail to parse perfectly valid code and thus provide empty or
+partial check reports. This option is useful when developers have no control over this
+third-party code and want to perform analysis at least on project sources.
+
+Another option for excluding third-party packages is :option:`pio check --skip-packages`
+command.
+
+**Example**
+
+.. code-block:: ini
+
+    [env:extra_check_flags]
+    platform = ...
+    board = ...
+    check_tool = cppcheck, clangtidy
+    check_skip_packages = yes