Enforce minimum cpu and memory limits (#731)

* add validation for PG resources and volume size
* check resource requests also on UPDATE and SYNC + update docs
* if cluster was running don't error on sync

docs/user.md

@@ -30,7 +30,7 @@ spec:
   databases:
     foo: zalando
   postgresql:
-    version: "10"
+    version: "11"
 ```
 
 Once you cloned the Postgres Operator [repository](https://github.com/zalando/postgres-operator)
@@ -40,6 +40,9 @@ you can find this example also in the manifests folder:
 kubectl create -f manifests/minimal-postgres-manifest.yaml
 ```
 
+Note, that the minimum volume size to run the `postgresql` resource on Elastic
+Block Storage (EBS) is `1Gi`.
+
 ## Watch pods being created
 
 ```bash
@@ -182,6 +185,32 @@ See [infrastructure roles secret](../manifests/infrastructure-roles.yaml)
 and [infrastructure roles configmap](../manifests/infrastructure-roles-configmap.yaml)
 for the examples.
 
+## Resource definition
+
+The compute resources to be used for the Postgres containers in the pods can be
+specified in the postgresql cluster manifest.
+
+```yaml
+apiVersion: "acid.zalan.do/v1"
+kind: postgresql
+metadata:
+  name: acid-minimal-cluster
+spec:
+  resources:
+    requests:
+      cpu: 10m
+      memory: 100Mi
+    limits:
+      cpu: 300m
+      memory: 300Mi
+```
+
+The minimum limit to properly run the `postgresql` resource is `256m` for `cpu`
+and `256Mi` for `memory`. If a lower value is set in the manifest the operator
+will cancel ADD or UPDATE events on this resource with an error. If no
+resources are defined in the manifest the operator will obtain the configured
+[default requests](reference/operator_parameters.md#kubernetes-resource-requests).
+
 ## Use taints and tolerations for dedicated PostgreSQL nodes
 
 To ensure Postgres pods are running on nodes without any other application pods,
@@ -305,7 +334,7 @@ Things to note:
 - There is no way to transform a non-standby cluster to a standby cluster
   through the operator. Adding the standby section to the manifest of a running
   Postgres cluster will have no effect. However, it can be done through Patroni
-  by adding the [standby_cluster] (https://github.com/zalando/patroni/blob/bd2c54581abb42a7d3a3da551edf0b8732eefd27/docs/replica_bootstrap.rst#standby-cluster)
+  by adding the [standby_cluster](https://github.com/zalando/patroni/blob/bd2c54581abb42a7d3a3da551edf0b8732eefd27/docs/replica_bootstrap.rst#standby-cluster)
   section using `patronictl edit-config`. Note that the transformed standby
   cluster will not be doing any streaming. It will be in standby mode and allow
   read-only transactions only.
@@ -384,7 +413,7 @@ specified but globally disabled in the configuration. The
 
 ## Increase volume size
 
-PostgreSQL operator supports statefulset volume resize if you're using the
+Postgres operator supports statefulset volume resize if you're using the
 operator on top of AWS. For that you need to change the size field of the
 volume description in the cluster manifest and apply the change: