Update User Guide to achieve mid-v1.1 milestone

[rlrh]

• [REQUIRED] Mark unimplemented features as 'Coming in v2.0'. See here for an example: https://github.com/nus-cs2103-AY1819S2/addressbook-level4/blob/master/docs/UserGuide.adoc#encrypting-data-files-code-coming-in-v2-0-code
• [LOW-PRIORITY] Update introduction with new copy text:
  • README helps you discover, curate and read more content you care about in less time. It is an integrated bookmark manager and feed reader desktop application for saving web pages and feeds to read later on your personal computer. You’ll love README if you’re a heavy consumer of web content but don’t have regular internet access, whether because you often head off on long flights or you love working in cafes, and refuse to pay exorbitant prices for Internet access anywhere. README is for those who prefer to use a desktop app...