[examples] Add statement at the beginning of autogenerated files

examples/1ptracer/README.md

+<!-- Important: This file has been automatically generated by generate_example_docs.py. Do not edit this file directly! -->
+
 # One-phase flow with random permeability distribution and a tracer model
 
 ## Problem set-up

examples/2pinfiltration/README.md

+<!-- Important: This file has been automatically generated by generate_example_docs.py. Do not edit this file directly! -->
+
 This tutorial is similar to tests/porousmediumflow/2p/adaptive and restricted to the cell-centered finite volume TPFA discretization scheme.
 You need [ALUGrid][0] in order to compile and run it.
 

examples/freeflowchannel/README.md

+<!-- Important: This file has been automatically generated by generate_example_docs.py. Do not edit this file directly! -->
+
 This example is based on dumux/test/freeflow/navierstokes/channel/2d.
 
 # Freeflow through a channel

examples/generate_example_docs.py

@@ -18,6 +18,8 @@ def convertToMarkdownAndMerge(dir, config):
         os.makedirs(os.path.dirname(targetPath), exist_ok=True)
 
         with open(targetPath, "w") as targetFile:
+            thisScript = os.path.basename(__file__)
+            targetFile.write("<!-- Important: This file has been automatically generated by {}. Do not edit this file directly! -->\n\n".format(thisScript))
             for source in sources:
                 fileExtension = os.path.splitext(source)[1]
                 if fileExtension == ".md":

examples/shallowwaterfriction/README.md

+<!-- Important: This file has been automatically generated by generate_example_docs.py. Do not edit this file directly! -->
+
 # Shallow water flow with bottom friction
 This example shows how the shallow water flow model can be
 applied to simulate steady subcritical flow including