update readme and explain what is first param for

in readme example:

```
var gulp = require('gulp');
var html2js = require('gulp-html2js');

gulp.task('default', function () {
gulp.src('templates/*.html')
.pipe(html2js('angular.js', {
adapter: 'angular',
base: 'templates',
name: 'angular-demo'
}))
.pipe(gulp.dest('dist/'));
});
```

What is first param 'angular.js' for?

I'm trying to read source and it doesn't seem to do anything it just checks if it's not empty and if it is it throw new PluginError(PLUGIN_NAME, 'Missing file option for gulp-html-to-js');

Start with the README example and then inspect the gulp-html2js source where the first argument is checked and the missing-file error is raised. Clarify the purpose of the 'angular.js' argument in the example, update the README, and verify that the explanation matches the plugin's behavior.

Written by the indexing model from the issue text.