Added description for converting an LE private key for acme-tiny.

README.md

@@ -34,6 +34,18 @@ this script likely isn't for you! Please use the official Let's Encrypt
 openssl genrsa 4096 > account.key
 ```
 
+#### Use existing Let's Encrypt key
+
+Alternatively you can convert your key, previously generated by the original
+Let's Encrypt client.
+
+The private account key from the Let's Encrypt client is saved in the
+[JWK](https://tools.ietf.org/html/rfc7517) format. `acme-tiny` is using the PEM
+key format. To convert the key, you can use the tool
+[pem-jwk](https://github.com/dannycoates/pem-jwk):
+
+    $ pem-jwk /etc/letsencrypt/accounts/acme-v01.api.letsencrypt.org/directory/<id>/private_key.json > account.key
+
 ### Step 2: Create a certificate signing request (CSR) for your domains.
 
 The ACME protocol (what Let's Encrypt uses) requires a CSR file to be submitted
@@ -182,4 +194,3 @@ anyone who wants to run it.
 
 If you want to add features for your own setup to make things easier for you,
 please do! It's open source, so feel free to fork it and modify as necessary.
-