Add DevCycleService to Nest SDK docs (#591)

docs/sdk/server-side-sdks/nestjs/nestjs-usage.md

@@ -35,9 +35,50 @@ export class MyController {
 }
 ```
 
+## DevCycle Service
+With the DevCycleModule imported, the `DevCycleService` can be injected into your controllers or providers. The DevCycleService methods evaluate variables with the user returned from your [userFactory](/sdk/server-side-sdks/nestjs/nestjs-gettingstarted#user-factory), so you don't need to specify a user each time a method is called.
+
+```typescript
+import { DevCycleService } from '@devcycle/nestjs-server-sdk'
+
+export class MyService {
+  constructor(
+    private readonly devcycleService: DevCycleService,
+  ) {}
+
+  async update() {
+    const enabled = this.devcycleService.isEnabled('allow-feature-edits')
+    if (enabled) {
+      // do something
+    }
+  }
+}
+```
+
+### variableValue
+The `variableValue` method accepts a variable key and default value, and returns the served value.
+
+```typescript
+const value = this.devcycleService.variableValue('variable-key', 'hello world')
+```
+
+### isEnabled
+The `isEnabled` method accepts a key for a boolean variable. The default value is always `false` when using the `isEnabled` method.
+
+```typescript
+const enabled = this.devcycleService.isEnabled('boolean-variable')
+```
+
+### getUser
+The `getUser` method returns the user object from your [userFactory](/sdk/server-side-sdks/nestjs/nestjs-gettingstarted#user-factory).
+
+```typescript
+const devcycleUser = this.devcycleService.getUser()
+```
+
 ## Decorators
 
-DevCycle decorator evaluate variables with the user returned from your [userFactory](/sdk/server-side-sdks/nestjs/nestjs-gettingstarted#user-factory), so you don't need to specify a user each time a decorator is used.
+DevCycle decorators evaluate variables with the user returned from your [userFactory](/sdk/server-side-sdks/nestjs/nestjs-gettingstarted#user-factory), so you don't need to specify a user each time a decorator is used.
 
 ### VariableValue
 The `VariableValue` decorator can be used to access variable values directly in your route handlers.