Update README.md

Author: piyalidas10

README.md

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 <details>
 
+<summary><strong>Angular Dependency Injection</strong></summary>
+
+### Why use Inject instead of DI with the Constructor ?
+Ans. This is class-based injection — Angular automatically provides the service instance when the component (or directive, or service) is created.
+✅ Works great for components, directives, pipes, and services.
+❌ But only works inside class constructors.
+If you want to use a service in a function, effect, or utility file, you can’t — because there’s no class context.
+
+</details>
+
+<details>
+
 <summary><strong>How performace is increased from Angular 2 to Latest</strong></summary>
 
 Let’s go deep into how Angular’s performance improved — from Angular 2 → Angular 19 (2025) — across rendering, build, memory, SSR, and runtime execution.