Tweak documentation to specify inclusion order of es5-shim and es6-shim

[jibesh]

For using in the browser, the documentation says 'it's strongly recommended to always include es5-shim' but it does not specify the order of inclusion. A side effect of not doing so is as follows:

• Only using es6-shim, the identifier returnExports points to an Object.
• If es5-shim is included before es6-shim, the identifier returnExports still points to an Object.
• But if es5-shim is included after es6-shim, returnExports becomes undefined. As a result, I would like to suggest a tweak in the documentation that specifies an inclusion order.

[ljharb]

Very good point, a PR would be welcome.

(That said, nobody should be using returnExports, because it's a shim - it's meant to simply install things on the environment)