Create README.md for CShellEngine project

Add README.md with project overview and usage example

Author: ssh-lvl

README.md

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+# CShellEngine
+CShellEngine is a framework library designed to help set up the boilerplate for C-based terminal games and applications.
+
+## Features
+- 3D/2D grid system.
+- Rendering of grid system.
+- Keyboard input handling.
+- Random number generation.
+- Game loop handling.
+- Delta time for game loop system.
+- Datatypes such as: `Vector2i`, `Vector3i`, `Vector2f`, `Vector3f`.
+
+## Platforms
+``` diff
++ Linux: Tested, supported.
+- Windows: Untested, currently unsupported.
+! macOS: Untested, expected to work (may require specific terminal settings/environment).
+```
+
+## Usage
+Simply include the headers you need in your C program.  
+And make sure to include all C source files in compilation.
+
+## Example
+``` c
+#include "Core.h"
+#include "Rendering.h"
+#include "Input.h"
+
+void game_loop() {
+    printf("main method.\n");
+    Core___game_loop_quit_flag = true;
+}
+
+void render() {
+    Rendering___render(NULL);
+}
+
+int main() {
+    Vector3i grid_size_ = {3, 1, 2};
+    Core___initCore(&grid_size_, &game_loop, NULL);
+    Rendering___init(NULL);
+
+    Input___setInputMode(false, false);
+    Rendering___setMapping(1, "Hello World!\n");
+
+    Core___grid[0][0][0] = 1;
+
+    while (!Input___isKeyPressed('a')); // wait for 'a' key
+
+    printf("\na pressed\n");
+    printf("%c pressed\n", Input___getKey()); // wait for and print pressed key
+
+    Core___startGameLoop(&render);
+
+    printf("\ndelta time: %f\nrender time: %f\n", Core___delta_time, Rendering___render_time);
+
+    Rendering___cleanup(NULL);
+    Core___cleanupCore(NULL);
+    Input___setInputMode(true, true);
+
+    return 0;
+}
+```