Add README.md

README.md

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+# PM3 Client Builder
+
+Build multiple clients of [Proxmark3 RRG repo](https://github.com/RfidResearchGroup/proxmark3)(for Windows) in parallel by GitHub Action.  
+
+## Usage
+1. Fork this repo  
+2. Go to `Actions` then enable it.  
+(If you don't want to affect your contribution chart, you can create a new branch then do the following steps)  
+3. Change the `config.json`  
+4. Push a commit to this repo or click `Run workflow` in `Actions`  
+5. Check and download the firmwares in `Actions`->the latest workflow run->`Artifacts`  
+
+## Examples of `config.json`
+
+### Build 3 versions on 4 platforms 
+
+<details>
+<summary>config.json</summary>
+
+
+```
+{
+    "refs": [
+        "master",
+        "v4.16191",
+        "v4.15864"
+    ],
+    "platforms": [
+        "PM3RDV4",
+        "PM3GENERIC",
+        "RDV4BTADDON",
+        "GENERICWITHFLASH"
+    ]
+}
+```
+
+</details>