MDL-28929 document themes api changes

diff --git a/theme/upgrade.txt b/theme/upgrade.txt
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+This files describes API changes in /theme/* themes,
+information provided here is intended especially for theme designer.
+
+=== 2.2 ===
+
+required changes:
+* use new page content placeholder "echo $OUTPUT->main_content()" instead of "echo core_renderer::MAIN_CONTENT_TOKEN"
+  see git commit: 3b3f302855d7621405a8b93e49bd399d67a998d7
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