Update 4 years later (Oct. 2013)
javabeangrinder's answer (upvoted) mentions the following trick:
To create a package-info.java file in an existing package:
- Right click on the package where you want a
package-info.java.
- Select
new->package.
- Check the
Create package.info.java check box.
- Click on
Finish
Original answer (May 2009)
There is no template or wizard to easily create a package.html file.
As mmyers said in his comment, since Java1.5, the correct file to create would be package-info.java.
That file can be used not only for javadocs, but also for package-level annotations (as illustrated here).
There is an opened Bug (#86168) for demanding a wizard for the creation of package-info.java (since the class wizard does not allow the package-info name).
The reflections on that topic are on since... 2005! The problem is that any solution should
be implemented in a way that it also helps with 1.4 code.
The current workaround is to create a package-info.java as a text file.
From that point forward, package-info.java behaves as a normal Java class, and Eclipse places a package-info.class file in the output folder.
The Javadocs are correctly built using package-info.java, and not the package.html file.

(source: developpez.com)
Note (in response to Strawberry's comment):
both wish a preview of the package overview in package-info.java in the Javadoc
view.
So far, no patch has been proposed/implemented.
There is simply not enough demands for that feature to be added.