Updated docs and added good/bad example yaml

Signed-off-by: Joaquin Fernandez Campo <jfcampo@gmail.com>

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## unreleased / ????-??-??
 
+* [ENHANCEMENT] Updated readme docs and added a test for invalid yaml ([#274](https://github.com/prometheus/statsd_exporter/issues/274))
 * [ENHANCEMENT] Support sampling factors for all statsd metric types ([#264](https://github.com/prometheus/statsd_exporter/issues/250))
 * [ENHANCEMENT] Support Librato and InfluxDB labeling formats ([#267](https://github.com/prometheus/statsd_exporter/pull/267))
 

README.md

@@ -200,6 +200,16 @@ could be written as:
 
 ```yaml
 mappings:
+- match: "test\\.(\\w+)\\.(\\w+)\\.counter"
+  match_type: regex
+  name: "${2}_total"
+  labels:
+    provider: "$1"
+```
+
+Be aware about yaml escape rules as a mapping like the following one will not work. 
+```yaml 
+mappings:
 - match: "test\.(\w+)\.(\w+)\.counter"
   match_type: regex
   name: "${2}_total"

pkg/mapper/mapper_test.go

@@ -842,6 +842,31 @@ mappings:
 			configBad:      true,
 			expectedAction: ActionTypeDrop,
 		},
+		{
+			// valid yaml example
+			config: `---
+mappings:
+- match: "test\\.(\\w+)\\.(\\w+)\\.counter"
+  match_type: regex
+  name: "${2}_total"
+  labels:
+    provider: "$1"
+`,
+			configBad:      false,
+			expectedAction: ActionTypeMap,
+		},
+		{
+			// invalid yaml example
+			config: `---
+mappings:
+- match: "test\.(\w+)\.(\w+)\.counter"
+  match_type: regex
+  name: "${2}_total"
+  labels:
+    provider: "$1"
+`,
+			configBad: true,
+		},
 	}
 
 	for i, scenario := range scenarios {