Add caveat about using regular with NEJ

[fengzilong]

because NEJ doesn't support importing umd library directly, we have to use regular in this way

define( [
    'path/to/regular'
], function ( R ) {
   Regular.extend( ... )
} )

Is it necessary to add some caveat in regular guide about this?

@AndyRightNow said he can provide some help on this ( PR )

head to regularjs/regular#150 for details

feel free to close this issue if we don't want this ;)