Readme copyediting

[reedy]

Hello! there seems to be a couple of missing words in your Readme and it sounds a bit odd :)

The polyfills slow down your application by adding function_exists, extension_loaded or php version checks that
are when your autoloader is loaded. By removing the polyfills the start up time is (slightly) reduced.

I think that should be "checks that are run when" not "checks that are when"

If you do add this or one of its child packages, consider adding extension as a requirement to your composer.json.

Should this be (adding a this), and possibly extension replaced with package?

If you do add this or one of its child packages, consider adding this extension as a requirement to your composer.json.