Add .gitattributes (prevent eol conversion on Windows) (#386)

Copy the .gitattributes from x/net.

.gitattributes

diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..d2f212e
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,10 @@
+# Treat all files in this repo as binary, with no git magic updating
+# line endings. Windows users contributing to Go will need to use a
+# modern version of git and editors capable of LF line endings.
+#
+# We'll prevent accidental CRLF line endings from entering the repo
+# via the git-review gofmt checks.
+#
+# See golang.org/issue/9281
+
+* -text