Memory check resource definition is not valid in README

[rgeniesse]

The README currently has the following for a memory check resource definition example:

---
type: CheckConfig
spec:
  command: "check-memory.rb"
  handlers: []
  high_flap_threshold: 0
  interval: 10
  low_flap_threshold: 0
  publish: true
  runtime_assets:
  - sensu-plugins/sensu-plugins-memory-checks
  - sensu/sensu-ruby-runtime
  subscriptions:
  - linux

This isn't valid for a few reasons:

• No metadata, so no check name can be set.
• Missing the api_version attribute.
• The memory check command throws an error that both warning and critical need to be set.
  The following worked for me:

---
type: CheckConfig
api_version: core/v2
metadata:
  name: check-memory
  namespace: default
spec:
  command: "check-memory.rb -w 80 -c 90"
  high_flap_threshold: 0
  interval: 10
  low_flap_threshold: 0
  publish: true
  runtime_assets:
  - sensu-plugins/sensu-plugins-memory-checks
  - sensu/sensu-ruby-runtime
  subscriptions:
  - linux