update README with env variables

testnet-faucet/.env.sample

@@ -0,0 +1,4 @@
+validatorAddress=
+mnemonic=
+testnetURL1=
+accountAddress=

testnet-faucet/README.md

@@ -4,6 +4,10 @@
 
 Run `yarn install`
 
+### Add environment variables
+
+Create a `.env` file in the root of the project. Copy the keys from the `.env.sample` file (found in the project root) into the `.env` file and and in the required values`
+
 ### Run development mode
 
 Run `yarn dev`