Add README with description and feature list

Author: leptos-null

README.md

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+## HeaderViewer
+
+HeaderViewer is an app for viewing Objective-C class and protocol headers generated by [ClassDumpRuntime](https://github.com/leptos-null/ClassDumpRuntime).
+
+This project was inspired by https://github.com/nst/RuntimeBrowser
+
+### Features
+
+- List all loaded classes and protocols
+- Filter class or protocol list by name
+- Syntax-colored header files
+- Navigate to class or protocol referenced in header
+- Browse system images
+    - Load image if not yet loaded
+    - View classes in each image