Improve security readme
```
**Securing API with Oauth2 authentication**
- i. Create a secret with the certificate of the wso2am server
+ i. Create a secret with the certificate of the API Portal
`
kubectl create secret generic <secret name> -n <namespace> --from-file=<path to cert>
```
- sample security definitions are provided in ./deploy/sample-definitions/security_definitions.yaml
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+ sample security definitions are provided in [here](../../apim-operator/deploy/sample-definitions/security_definitions.yaml)
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