Add readme for SVG generation

Author: linev

test/svg_ref/Readme.md

@@ -0,0 +1,17 @@
+# Reference SVG files for stressGraphics tests
+
+Used for `test-stressgraphics-svg` test, to run:
+
+    ctest -R test-stressgraphics-svg
+
+
+If test failing while code is changed and reference files need to be updated,
+run stressGraphics with following arguments:
+
+    ./stressGraphics -k --web=off -p=svg --svg=/home/user/git/root/test/svg_ref/
+
+`-k` in the arguments list allow to overwrite reference files so one can directly
+create PR with updated files.
+
+But before updating reference file - please check changed images.
+Maybe produce graphics is wrong and one better need to fix code which produce it.