fixing the readme to indicate err return
from a test that I wrote in https://github.com/arschles/go-bindata-test, it looks like go-bindata generates an Asset(...) function that returns an error when the file isn't found
Merge pull request #54 from arschles/fix-readme

fixing the readme to indicate err return
Fix Asset signature in README.

See https://github.com/jteeuwen/go-bindata/pull/54#discussion_r20400279.
Merge pull request #55 from shurcooL/patch-1

Fix Asset signature in README.