gitignore: ignore "docs" and "vendor"

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

.gitignore

@@ -1,6 +1,8 @@
 /agent-*
 /coverage
 /covdir
+/docs
+/vendor
 /gopath
 /gopath.proto
 /go-bindata
@@ -16,4 +18,4 @@
 *.test
 hack/tls-setup/certs
 .idea
-*.bak
+*.bak