Add general run instructions

[camillobruni]

Let's collect some general (and browser specific) instructions to get consistent Speedometer 3 scores across browsers and platforms.

WIP google document to collect information.

[bgrins]

Based on the discussion today here, the likely outcome is

1. Some text to replace the current text on the homepage (Speedometer is a browser benchmark that measures the responsiveness of Web applications. It uses demo web applications to simulate user actions such as adding to-do items.)
2. A more detailed runbook for people who want to report the most accurate scores

[rniwa]

Yeah, I guess it would be good to point out things like clearing the cache, disabling all browser extensions, etc... for the detailed instructions.