Update README.md (#57)

* Update README.md

If your gcloud on the server isn't authenticated for your Google Cloud, you'll get an error that looks something like:
```
ERROR: (gcloud.compute.disks.list) Some requests did not succeed:
 - Insufficient Permission
```

* Update README.md

README.md

@@ -19,6 +19,7 @@ gcloud-snapshot.sh will:
 
 	[	http://stackoverflow.com/questions/31905966/gcloud-compute-list-networks-error-some-requests-did-not-succeed-insufficie#31928399](http://stackoverflow.com/questions/31905966/gcloud-compute-list-networks-error-some-requests-did-not-succeed-insufficie#31928399)
 * The version of gcloud is up to date: `gcloud components update` 
+* You are authenticated with gcloud - normally this happens automatically, but some users get the error "Insufficient Permission" and need to authenticate: `sudo gcloud auth login`
 
 ## Installation
 
@@ -125,4 +126,4 @@ Permission is hereby granted, free of charge, to any person obtaining a copy of
 
 The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
 
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.