Common: mention the contributing guide in the readme (closes #4007)
Common: mention the contributing guide in the readme (closes #4007)

liveintent-berlin/snowplow,liveintent-berlin/snowplow,RetentionGrid/snowplow,snowplow/snowplow,snowplow/snowplow,RetentionGrid/snowplow,snowplow/snowplow,liveintent-berlin/snowplow,liveintent-berlin/snowplow,snowplow/snowplow,RetentionGrid/snowplow,liveintent-berlin/snowplow,RetentionGrid/snowplow,RetentionGrid/snowplow,liveintent-berlin/snowplow