[STK-322] Add How it works doc (#12)

* Add How it works doc

* Update How-it-works.md

* Update how it works

* Add how it works reference in Readme

docs/How-it-works.md

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+# How it works?
+
+Reloader watches for `ConfigMap` and `Secret` and detects if there are changes in data of these objects. After change detection reloader performs rolling upgrade on relevant Pods via associated `Deployment`, `Deamonset` and `Statefulset`.
+
+## How change detection works
+
+Reloader watches changes in `configmaps` and `secrets` data. As soon as it detects a change in these. It forwards these objects to an update handler which decides if and how to perform the rolling upgrade.
+
+## Requirements for rolling upgrade
+
+To perform rolling upgrade a `deployment`, `daemonset` or `statefulset` must have
+
+- support for rolling upgrade strategy
+- specific annotation for `configmaps` or `secrets`
+
+The annotation value is comma separated list of `configmaps` or `secrets`. If a change is detected in data of these `configmaps` or `secrets`, reloader will perform rolling upgrades on their associated `deployments`, `daemonsets` or `statefulsets`.
+
+### Annotation for Configmap
+
+For a `Deployment` called `foo` have a `ConfigMap` called `foo`. Then add this annotation to your `Deployment`
+
+```yaml
+metadata:
+  annotations:
+    configmap.reloader.stakater.com/reload: "foo"
+```
+
+### Annotation for Secret
+
+For a `Deployment` called `foo` have a `Secret` called `foo`. Then add this annotation to your `Deployment`
+
+```yaml
+metadata:
+  annotations:
+    secret.reloader.stakater.com/reload: "foo"
+```
+
+Above mentioned annotation are also work for `Daemonsets` and `Statefulsets`
+
+## How Rolling upgrade works?
+
+When reloader detects changes in configmap. It gets two objects of configmap. First object is an old configmap object which has a state before the latest change. Second object is new configmap object which contains latest changes. Reloader compares both objects and see whether any change in data occurred or not. If reloader finds any change in new configmap object, only then, it move forward with rolling upgrade.
+
+After that, reloader gets the list of all deployments, daemonsets and statefulset and looks for above mentioned annotation for configmap. If the annotation value contains the configmap name, it then looks for an environment variable which can contain the configmap or secret data change hash.
+
+### Environment variable for Configmap
+
+If configmap name is foo then
+
+```yaml
+STAKATER_FOO_CONFIGMAP
+```
+
+### Environment variable for Secret
+
+If Secret name is foo then
+
+```yaml
+STAKATER_FOO_SECRET
+```
+
+If the environment variable is found then it gets its value and compares it with new configmap hash value. If old value in environment variable is different from new hash value then reloader updates the environment variable. If the environment variable does not exist then it creates a new environment variable with latest hash value from configmap and updates the relevant `deployment`, `daemonset` or `statefulset`
+
+Note: Rolling upgrade also works in the same way for secrets.
+
+### Hash value Computation
+
+Reloader uses SHA1 to compute hash value. SHA1 is used because it is efficient and less prone to collision.
+
+## Monitor All Namespaces
+
+By default reloader deploys in default namespace and monitors changes in all namespaces. To monitor changes in a specific namespace deploy the reloader in that namespace and set the `watchGlobally` flag to `false` in values file located under `deployments/kubernetes/chart/reloader`
+And render manifest file using helm command
+```bash
+helm --namespace {replace this with namespace name} template . > reloader.yaml
+```
+The output file can then be used to deploy reloader in specific namespace.