Update README.md to indicate Go 1.7+ is required to use this package

New functionality added to this package now depends on the `context` package
that was introduced in Go 1.7. This change updates the README to reflect that
new version constraint.

If this becomes an issue, we could add the `go-constraint` package as a
dependency to this. This would allow us to have a compile time check of the Go
version.

README.md

@@ -9,7 +9,11 @@ includes a non-blocking TryLock() function to allow locking without blocking exe
 ## License
 `flock` is released under the BSD 3-Clause License. See the `LICENSE` file for more details.
 
-## Intsallation
+## Go Compatibility
+This package makes use of the `context` package that was introduced in Go 1.7. As such, this
+package has an implicit dependency on Go 1.7+.
+
+## Installation
 ```
 go get -u github.com/theckman/go-flock
 ```