contributor-guide: add notes for tests

This adds some hints that and how changes should be tested when contributing.

Fixes [YOCTO #15412]

(From yocto-docs rev: d6f14fad6b85e61961830198474f9281d84b3d27)

Signed-off-by: Simone Weiß <simone.p.weiss@posteo.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/contributor-guide/submit-changes.rst

@@ -221,6 +221,38 @@ to add the upgraded version.
    <https://www.kernel.org/doc/html/latest/process/submitting-patches.html#using-reported-by-tested-by-reviewed-by-suggested-by-and-fixes>`__
    in the Linux kernel documentation.
 
+Test your changes
+-----------------
+
+For each contributions you make, you should test your changes as well.
+For this the Yocto Project offers several types of tests. Those tests cover
+different areas and it depends on your changes which are feasible. For example run:
+
+   -  For changes that affect the build environment:
+
+      -  ``bitbake-selftest``: for changes within BitBake
+
+      -  ``oe-selftest``: to test combinations of BitBake runs
+
+      -  ``oe-build-perf-test``: to test the performance of common build scenarios
+
+   -  For changes in a recipe:
+
+      - ``ptest``: run package specific tests, if they exist
+
+      - ``testimage``: build an image, boot it and run testcases on it
+
+      - If applicable, ensure also the ``native`` and ``nativesdk`` variants builds
+
+   -  For changes relating to the SDK:
+
+      - ``testsdk``: to build, install and run tests against a SDK
+
+      - ``testsdk_ext``: to build, install and run tests against an extended SDK
+
+Note that this list just gives suggestions and is not exhaustive. More details can
+be found here: :ref:`test-manual/intro:Yocto Project Tests --- Types of Testing Overview`.
+
 Creating Patches
 ================
 
@@ -285,8 +317,9 @@ Validating Patches with Patchtest
 ``patchtest`` is available in ``openembedded-core`` as a tool for making
 sure that your patches are well-formatted and contain important info for
 maintenance purposes, such as ``Signed-off-by`` and ``Upstream-Status``
-tags. Currently, it only supports testing patches for
-``openembedded-core`` branches. To setup, perform the following::
+tags. Note that no functional testing of the changes will be performed by ``patchtest``.
+Currently, it only supports testing patches for ``openembedded-core`` branches.
+To setup, perform the following::
 
     pip install -r meta/lib/patchtest/requirements.txt
     source oe-init-build-env