Add FAQ.md file/wiki with some common questions/answers?

[pdehaan]

/FAQ.md

"How big of a file can I transfer with Firefox Send" (See #176 (comment))

"Why is my browser not supported?" (See #101 (comment))

"How long are files available for?" — One download or 24 hours, whichever comes first.

"Can a file be downloaded more than once?" — No.

[ghost]

draft @ https://docs.google.com/document/d/1gk2Z2jw6CLivHld9gCGZzCX102d_G--20VHEX6CJ3zo/edit?usp=sharing

r? @pdehaan @johngruen

[pdehaan]

Sorry, I commented on the goog doc, but didn't follow up here...
The FAQ looks awesome 👍. Pending any further feedback from Mr. @johngruen, I think it's safe to PR it and we can merge+tweak as needed.