Add README with installation and usage instructions

README.md

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+# GhidraNes
+
+A Ghidra extension to support disassembling and analyzing NES ROMs.
+
+![NES game disassembly](.github/screenshots/ghidra-nes.png)
+
+## Features
+
+- Import NES ROMs in the iNES format. The following mappers are supported:
+    - NROM (mapper 0)
+- Add labels and memory blocks in disassembly, making it easier to jump around a disassembled ROM!
+
+## Installation
+
+1. Install a Compatible version of Java and Ghidra (Java 11+, tested with Ghidra 9.1.2).
+2. Download the latest [GhidraNes release](https://github.com/kylewlacy/GhidraNes/releases).
+3. Go to "File" > "Install Extensions...". Click "+" in the top-right corner and choose the GhidraNes Zip. Click "OK" to install the extension.
+4. Restart Ghidra.
+
+## Usage
+
+1. In Ghidra, create a new project by following the wizard under "File" > "New Project...".
+2. Drag-and-drop an iNES `.nes` ROM onto the project. Set the format to "NES ROM" and click "OK".
+3. Double-click the ROM in the project to open Ghidra's CodeBrowser.
+4. Analyze the file when prompted (or go to "Analysis" > "Auto Analyze..."). Leave the settings as default and click "Analyze".
+5. Done, the game will be disassembled! On the left-hand side, under "Symbol Tree" > "Functions", open `reset` to jump to the reset vector (where execution starts), or `vblank` to jump to the NMI vector (where execution goes during VBlank).
+
+## Developing
+
+1. Install Java and Ghidra.
+2. install Eclipse.
+3. Install the GhidraDev Eclipse plugin. Instructions can be found in your Ghidra install directory, under `Extensions/Eclipse/GhidraDev/GhidraDev_README.html`.
+4. In Eclipse, open the GhidraNes repo by going to "File" > "Open Projects from File System...".
+5. Open "GhidraDev" > "Link Ghidra...". Add your Ghidra installation, click "Next >", then select the "GhidraNes" as the Java project. Click "Finish".
+6. Run the project in Eclipse to start Ghidra and the GhidraNes extension.
+
+### Building a release from Eclipse
+
+**NOTE:** Ensure the GhidraNes Eclipse project is set up with the _earliest_ version of Java that should be targeted. Using a later version of Java can cause compatibility issues!
+
+1. Install Gradle (with [SDKMAN](https://sdkman.io/), this can be done with `sdk install gradle`).
+2. In Eclipse, open "GhidraDev" > "Export" > "Ghidra Module Extension...". Choose "GhidraNes" as the project, click "Next >", then choose "Local installation directory:" and browse to your Gradle installation dir (with SDKMAN, this will be at `~/.sdkman/candidates/gradle/$GRADLE_VERSION`). Click "Finish".
+3. The built zip file will be saved in the `dist/` directory.