Update golang/content.md to hint about git-lfs (#2389)

* Update golang/content.md to explain how to install git-lfs

The Go toolchain uses the local git installation to fetch archives of modules from Git servers directly. My company commits large files using Git LFS which happen to be part of our Go module source tree. Without _git-lfs_ installed, the local git installation does not automatically resolve those files which causes "_checksum mismatch_" against the committed `go.sum`.

Accept the more succinct version suggested in the pull-request.

golang/content.md

@@ -69,3 +69,7 @@ $ for GOOS in darwin linux; do
 >   done
 > done
 ```
+
+## Git LFS
+
+If downloading your dependencies results in an error like "checksum mismatch", you should check whether they are using [Git LFS](https://git-lfs.com/) (and thus need it installed for downloading them and calculating correct `go.sum` values).