Add resource translation readme file.

RESOURCE.md

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+# RESOURCES
+
+Stackdriver has defined [resource types](https://cloud.google.com/monitoring/api/resources) for monitoring and for each resource type there
+are mandatory resource labels. OpenCensus has defined [standard resource](https://github.com/census-instrumentation/opencensus-specs/blob/master/resource/StandardResources.md)
+types and labels. 
+This document describes the translation from OpenCensus resources to Stackdriver resources
+performed by this exporter.
+
+## Mapping between Stackdriver and OpenCensus Resources
+
+### k8s_container
+
+**condition:** resource.type == container
+
+*k8s_container takes a precedence, so GKE will be mapped to k8s_container and its
+associated resource labels but it will not contain any gcp_instance specific
+resource labels.*
+
+
+| Item                | OpenCensus         | Stackdriver    |
+|---------------------|--------------------|----------------|
+| **resource type**   | container          | k8s_container  |
+| **resource labels** |                    |                |
+|                     | cloud.zone         | location       |
+|                     | k8s.cluster.name   | cluster_name   |
+|                     | k8s.namespace.name | namespace_name |
+|                     | k8s.pod.name       | pod_name       |
+|                     | container.name     | container_name |
+
+
+### gcp_instance
+**condition:** cloud.provider == gcp
+
+| Item                | OpenCensus         | Stackdriver    |
+|---------------------|--------------------|----------------|
+| **resource type**   | cloud              | gcp_instance   |
+| **resource labels** |                    |                |
+|                     | host.id            | instance_id    |
+|                     | cloud.zone         | zone           |
+
+
+### aws_ec2_instance
+**condition:** cloud.provider == aws
+
+| Item                | OpenCensus         | Stackdriver      |
+|---------------------|--------------------|------------------|
+| **resource type**   |                    |                  |
+|                     | cloud              | aws_ec2_instance |
+| **resource labels** |                    |                  |
+|                     | host.id            | instance_id      |
+|                     | cloud.region       | region           |
+|                     | cloud.account.id   | aws_account      |
+