Clarify that web server is required run Riot

[jryans]

Apparently some self-hosted Riots had been getting away with running it locally from a file URL until recent changes broke this.

Since file URLs are regularly locked down by browsers, we should clarify that a web server of some kind serving over HTTP(S) is required and file URLs are not supported.

[t3chguy]

https://github.com/vector-im/element-web/#getting-started