Clarified escaping for filters (#116)

This came up several times, including #115.

Author: jkohen

README.md

@@ -51,11 +51,13 @@ stackdriver-prometheus-sidecar --help
 The `--filter` flag allows to provide filters which all series have to pass before being sent to Stackdriver. The flag may be repeated to provide several filters. Filters use the same syntax as the well-known PromQL label matchers, e.g.:
 
 ```
-stackdriver-prometheus-sidecar --filter='job="k8s"' --filter='__name__!~"cadvisor_.+"' ...
+stackdriver-prometheus-sidecar --filter=job="k8s" --filter=__name__!~"cadvisor_.+" ...
 ```
 
 This drops all series which do not have a `job` label `k8s` and all metrics that have a name starting with `cadvisor_`.
 
+Note: On the command-line shell you may need to escape filters as `--filter='job="k8s"'`.
+
 #### File
 
 The sidecar can also be provided with a configuration file. It allows to define static metric renames and to overwrite metric metadata which is usually provided by Prometheus. A configuration file should not be required for the majority of users.