[w3c/manifest] Describe how developers can block web crawlers (#498)

There may be a case where a web app developer does not want their web app to be crawled.  I was thinking proposing an additional element to support this, but realized the web already has a way to block crawlers.  

Below is some draft language, if this seems reasonable, please 👍 and I'll submit a PR.

> Like other web resources, a web app manifest should be accessible to any web browser or web crawler.
>  
> If a web app developer wants to block traffic from web crawlers, the developer MAY do so by including the web app manifest in a robots.txt file.  This is further described in the [robots.txt](http://www.robotstxt.org/) protocol.

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