Changed the name of myCount.js and added sumfield.js

myCount.js is now called action-storecount.js

sumfield.js adds the sumfield widget that takes a filter and sums up the specified field in the resulting list of tiddlers and stores this value in another speficied field and tiddler.
Updated the README.md to include the sumfield widget

The sumfield widget needs to be changed somehow so that it doesn't constantly update.

I don't know how to do this.