docs: Add few notes regarding html plugin being optional

The html output plugin is optional and not installed by default. Let's
increase the visibility of the ways to make it working.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

docs/source/GetStartedGuide.rst

@@ -131,9 +131,10 @@ attempt to do so by running::
 
   pip install -r https://raw.githubusercontent.com/avocado-framework/avocado/master/requirements.txt
 
-The result, though, is highly dependent on your system setup, such as
-having the right compilers, header files and libraries available.  The
-more predictable and complete Avocado experience can be achieved with
+This way you only get the base avocado-framework without the optional
+plugins. Additionally the installation requires correctly configured
+system with the right compilers, header files and libraries available.
+The more predictable and complete Avocado experience can be achieved with
 the official RPM packages.
 
 Using Avocado

docs/source/ResultFormats.rst

@@ -54,6 +54,13 @@ You can also request that the report be opened automatically by using the
 Will show you the nice looking HTML results report right after ``sleeptest``
 finishes running.
 
+.. warning:: The HTML output is an optional plugin and has to be installed
+             separately as ``avocado-plugins-output-html`` RPM or by executing
+             ``cd optional_plugins/html && python setup.py install``
+             from avocado sources. Note it's enabled by default when using
+             avocado from sorces by ``make develop`` or ``make link``.
+
+
 Machine readable results
 ------------------------