[readme] Document official mission (closes #3)

[joshlf]

Let's use this PR as a place to workshop the wording of our mission and goals. The current version of this PR is taken from the original announcement, and should be thought of merely as a first draft.

[stusmall]

Should we expand: "It should be clear how to programmers how to use security-sensitive APIs" to "It should be clear how to programmers how to perform security-sensitive tasks"

This will include the original goal and expand it to include common tasks password storage, authentication, etc instead of just APIs

[joshlf]

@stusmall Done.

[joshlf]

FCP, so to speak: I'll merge this later today unless there are any objections.

[tarcieri]

@joshlf I YOLO'd my only suggestion myself 😉