Update README to specify you must do ng build at root level before npm i in sample/modular

[crlang44]

If you try to npm i in samples/modular you get the following error:
npm ERR! code ENOLOCAL npm ERR! Could not install from "../../dist/packages-dist" as it does not contain a package.json file.

Update the README at the root level to include the instruction to do ng build at the root level before trying to run the sample.

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