add e2e tests for postcss-cli (#797)

* add e2e tests for postcss-cli

* add readme for e2e

Author: romainmenke

e2e/README.md

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+# e2e tests
+
+We try to standardize how we build, bundle and publish each plugin.
+To verify that we are doing this correctly for the ecosystem as a whole we have a set of e2e tests.
+
+Things we currently verify :
+- can we use plugins with `postcss-loader`?
+- can we bundle plugins with `webpack` and use them in a browser?
+- can we use TypeScript and import/run plugins?
+- can we use plugins with `postcss-cli`?
+
+We want to be sure that any give change is safe for as many users as possible.
+
+_Contributions are welcome._