Add install instructions to README

README.md

 # Simple offline Gnosis Safe signer
 
-Usage:
+## Install
+
+```
+git clone https://gitlab.com/bingen/gnosis-offline-signer.git
+cd gnosis-offline-signer
+python3 -m venv env
+source env/bin/activate
+pip3 install -r requirements.txt
+```
+
+## Usage:
 
 ```
 python3 ./safe_sign.py -s <your-safe-tx-hash> -k <your-wallet-keystore-file>