add FAQ. Fixes mozilla#186

docs/faq.md

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+## How big of a file can I transfer with Firefox Send?
+
+There is a 2GB file size limit built in to Send, however, in practice you may
+be unable to send files that large.  Send encrypts and decrypts the files in
+the browser which is great for security but will tax your system resources.  In
+particular you can expect to see your memory usage go up by at least the size
+of the file when the transfer is processing.  You can see [the results of some
+testing](https://github.com/mozilla/send/issues/170#issuecomment-314107793).
+For the most reliable operation on common computers, it’s probably best to stay
+under a few hundred megabytes.
+
+## Why is my browser not supported?
+
+We’re using the [Web Cryptography JavaScript API with the AES-GCM
+algorithm](https://www.w3.org/TR/WebCryptoAPI/#aes-gcm) for our encryption.
+Many browsers support this standard and should work fine, but some have not
+implemented it yet (mobile browsers lag behind on this, in
+particular).
+
+## How long are files available for?
+
+Files are available to be downloaded for 24 hours, after which they are removed
+from the server.  They are also removed immediately after a download completes.
+
+## Can a file be downloaded more than once?
+
+Not currently, but we're considering multiple download support in a future
+release.
+
+
+*Disclaimer: Send is an experiment and under active development.  The answers
+here may change as we get feedback from you and the project matures.*