docs(build): documented environment support

docs/ENVIRONMENTS.md

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+# Environments support 
+
+## Environment.ts
+
+In the `src/environments/model.ts` is provided the Environment interface as follows
+
+```
+...
+export interface Environment {
+	production: boolean;
+	hmr: boolean;
+	...
+```
+
+`production`, which is a boolean, will specify is your application runs under a production environment, 
+while `hmr` will indicate if it runs under a Hot Module Replacement environment. 
+
+This interface will then be defined by default in `src/environments/environment.ts`: 
+```
+...
+export const environment: Environment = {
+	production: false,
+	hmr: false,
+	showDevModule: true,
+	...
+```
+
+## How to find out which environment is your application is currently using? 
+
+All you have to do is to import the environment.ts constant anywhere you want in your application: 
+```
+import { environment } from "environments/environment";
+```
+
+This way, you will be able to programmatically determine which environment is used in your app
+with simple checks, as follows: 
+
+```
+environment.production 
+// if the output of this line is true, we are in production environment. 
+// Otherwise, we are in hmr environment. 
+```
+
+## How to add a new environment ?
+
+First, create you new Ts file in the `src/environments` folder. 
+
+Then, make sure your new environment extends the `src/environments/model.ts` interface.
+
+Finally, define the options and file replacement of your new environment in the `angular.json` file. 
+
+For exemple, the environment.dummyEnv.ts file: 
+
+````
+import { enableProdMode, NgModuleRef } from "@angular/core";
+import { disableDebugTools } from "@angular/platform-browser";
+import { Environment } from "./model";
+
+enableProdMode();
+
+export const environment: Environment = { 
+
+	production: false,
+	hmr: false,
+
+	decorateModuleRef(modRef: NgModuleRef<any>): NgModuleRef<any> {
+		disableDebugTools();
+		return modRef;
+	},
+	ENV_PROVIDERS: []
+}
+````
+
+And in the angular.json file: 
+
+````
+ "dummyEnv" : {
+    "fileReplacements": [
+      {
+        "replace": "src/environments/environment.ts",
+         "with": "src/environments/environment.dummyEnv.ts"
+      }
+    ]
+  }
+````