Move NEWS.md intro to the dev version section so pkgdown won't skip

`pkgdown` seems to skip material preceding the first version number heading, as
well as top-level headings that don't follow a certain format. Move the intro
material to somewhere near the top where it won't be skipped; we'll need to move
it again each time we release a version.

Author: lcbrooks

NEWS.md

@@ -1,9 +1,9 @@
+# epiprocess 0.5.0.9999 (development version)
+
 Note that `epiprocess` uses the [Semantic Versioning
 ("semver")](https://semver.org/) scheme for all release versions, but not for
 development versions. A ".9999" suffix indicates a development version.
 
-# epiprocess 0.5.0.9999 (development version)
-
 ## Cleanup:
 
 * Added a `NEWS.md` file to track changes to the package.