Bump version to 0.6.0

Changes since 0.5.1:
  * Drop py2 support.
  * Handle long tagline properly.
  * Add spell/ and colors/ subdirs to be crawled for doc blocks.
  * Fail for duplicate section ids.
  * Make @order smarter, not requiring implicit built-in sections to be
    re-declared and intelligently merging explicit and implicit orders.

Author: dbarnett

vimdoc/VERSION.txt

@@ -1 +1 @@
-0.5.1
+0.6.0