Unnecessary files in npm package

[mariotacke]

The 0.4.0 version of the package contains some unnecessary files which makes simpleheat as a dependency larger than it needs to be. For example, the demo directory (14kb) and yarn.lock file (27kb) are unnecessary in the released version.

You could easily exclude these by adding an .npmignore file into the root before packaging:

.npmignore
demo/
yarn.lock