Add Cluster Prerequisites to README (#68)

* Add cluster prerequisites
* Fix typo

Author: iyannsch

README.md

@@ -6,7 +6,16 @@ There are three charts:
 
 * `theia-cloud-base` installs cluster wide resources that may be used by multiple Theia Cloud installations
 * `theia-cloud-crds` installs the custom resource definitions
-* `theia-cloud` installs Theia Cloud itself and depends on `theia-cloud-base` and `theia-cloud-cds`
+* `theia-cloud` installs Theia Cloud itself and depends on `theia-cloud-base` and `theia-cloud-crds`
+
+## Cluster Prerequisites
+The charts depend on well-established software in the Kubernetes ecosystem. Please make sure to install the dependencies before releasing with _helm_.
+
+* **cert-manager.io** is used for certificate management, supports internal/testing issuers and supports Let's Encrypt certificates. Installation instructions can be found [here](https://cert-manager.io), a helm chart [here](https://cert-manager.io/docs/installation/helm/). 
+
+* **ingress-nginx** is used to performantly assemble _nginx_ configuration files and synchronizes changes. Learn more about it [here](https://kubernetes.github.io/ingress-nginx/).
+
+You can find more information in the official [Theia Cloud documentation](https://theia-cloud.io/documentation/setuptheiacloud/). 
 
 ## Versioning