Make the DWO installation documentation more visible

[akurinnoy]

Is your task related to a problem? Please describe

The README.md does not have links to the DWO installation documentation (https://github.com/devfile/devworkspace-operator/blob/main/docs/installation.md), which makes the documentation less visible or even hidden.

Describe the solution you'd like

Put installation.md to the README.md for better visibility.