Update Features.hs

[akhra]

Some attempts at better clarity on why pure/static is good, and changed a few "you can't" and "you must" to language that doesn't sound like it's placing a burden on the user.

[chrisdone]

Please see also:

• Clarify how to use types for more than validation #62
• Explain what type errors have to do with purity #65
• Simpler examples for type inference #68

I'm glad I held off on merging in anticipation of further attempts to rewrite the content.

Ping @gelisam

[gelisam]

This pull request improves the wording, while my pull requests improve the choice of examples. It would be a messy merge, but I think we can have both kinds of improvements if we want to.

Btw, I like the improvements to the wording, but I'm not convinced that the new IO explanation ("IO operations are described by what they are, not what they do") is any clearer than the current version.

[akhra]

Admittedly that was as much for aesthetics as clarity. "side-effecting" is hideous. ;)