Add document for SubscriptionConfig

This commit introduces a document that highlights how a user can
configure operators deployed via OLM.

Author: awgreene

doc/design/subscription-config.md

@@ -0,0 +1,34 @@
+# Subscription Config
+
+## Configuring Operators deployed by OLM
+
+It is possible to configure how OLM deploys an operator via the `config` field in the [subscription](../../pkg/api/apis/operators/subscription_types.go) object.
+
+Currently, the OLM supports the following configurations:
+
+### Env
+
+The `Env` field defines a list of environment variables that must exist in all containers in the `pod` created by OLM.
+
+> Note: Values defined here will overwrite existing environment varaibles of the same name.
+
+### EnvFrom
+
+The `EnvFrom` field defines a list of sources to populate environment variables in the container. 
+The keys defined within a source must be a C_IDENTIFIER. 
+All invalid keys will be reported as an event when the container is starting.
+When a key exists in multiple sources, the value associated with the last source will take precedence.
+
+> Note: Values defined by an Env with a duplicate key will take precedence.
+
+### Volumes
+
+The `Volumes` field defines a list of [volumes](https://kubernetes.io/docs/concepts/storage/volumes/) that must exist on the `pod` created by OLM.
+
+> Note: Volumes defined here will overwrite existing Volumes of the same name.
+
+### VolumeMounts
+
+The `VolumeMounts` field defines a list of [volumeMounts](https://kubernetes.io/docs/concepts/storage/volumes/) that must exist in all containers in the `pod` created by OLM. If a `volumeMount` references a `volume` that does not exist, OLM will fail to deploy the operator.
+
+> Note: VolumeMounts defined here will overwrite existing VolumeMounts of the same name.