docs: add installation instructions

Signed-off-by: Alexy Mantha <[email protected]>

README.md

@@ -1,3 +1,45 @@
 # argocd-progressive-sync-extensions
-ArgoCD UI extension to provide better feedback on the status of progressive syncs
 
+ArgoCD UI extension to provide better feedback on the status of progressive syncs. It adds a new box in the status panel when the current application is waiting for a progressive sync to progress.
+
+## Prerequisites
+
+This extension depends on the Progressive Sync alpha feature and needs to be enabled manually, see [docs](https://argo-cd.readthedocs.io/en/stable/operator-manual/applicationset/Progressive-Syncs/) for more information on how to enable and use the feature.
+
+## Installation
+The extension can be installed using the [ArgoCD Extension Installer init container](https://github.com/argoproj-labs/argocd-extension-installer). 
+
+Here is an example of a Kustomize patch to install the extension: 
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: argocd-server
+spec:
+  template:
+    spec:
+      initContainers:
+      - name: progressive-sync
+        image: quay.io/argoprojlabs/argocd-extension-installer:v0.0.1
+        env:
+        - name: EXTENSION_URL
+          value: https://github.com/alexymantha/argocd-progressive-sync-extension/releases/download/<VERSION>/extension.tar.gz
+        - name: EXTENSION_CHECKSUM_URL
+          value: https://github.com/alexymantha/argocd-progressive-sync-extension/releases/download/<VERSION>/checksum.txt
+        securityContext:
+          allowPrivilegeEscalation: false
+          runAsUser: 1000
+        volumeMounts:
+          - mountPath: /tmp/extensions/
+            name: extensions
+      # Also need to update the default container to add the volume mount and define the volume
+      containers:
+        - name: argocd-server
+          volumeMounts:
+            - name: extensions
+              mountPath: /tmp/extensions/
+      volumes:
+        - name: extensions
+          emptyDir: {}
+```
+**Replace <VERSION> with the desired extension version**