[manifest] Additional context for the Abstract (#392)

In the following discussion thread, Ben Francis makes some good points to consider including to the abstract, especially the following discoverability/crawler use case in posting 17 (although the later is mentioned in section 12):

[[
<https://groups.google.com/forum/#!topic/mozilla.dev.platform/RAHBNdesiXs> 

W3C manifests allow web apps to be crawled by search engines and discovered 
by users through their user agent without needing them to be submitted to a 
central app store by the developer. They provide the metadata needed to 
describe a web app, and provide a built-in discovery mechanism. 
]]

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/392