updated README to include OpenSSH comments (#161)

Author: a-maccormack

README.md

@@ -163,6 +163,26 @@ See the detail information about [SSH login without password](http://www.linuxpr
 * Change the permissions of `.ssh` to 700
 * Change the permissions of `.ssh/authorized_keys2` to 640
 
+### If you are using OpenSSH
+If you are currently using OpenSSH and are getting the following error:
+
+```bash
+ssh: handshake failed: ssh: unable to authenticate, attempted methods [none publickey]
+```
+
+Make sure that your key algorithm of choice is supported.
+On Ubuntu 20.04 or later you must explicitly allow the use of the ssh-rsa algorithm. Add the following line to your OpenSSH daemon file (which is either `/etc/ssh/sshd_config` or a drop-in file under 
+`/etc/ssh/sshd_config.d/`):
+
+```
+CASignatureAlgorithms +ssh-rsa
+```
+
+Alternatively, `ed25519` keys are accepted by default in OpenSSH. You could use this instead of rsa if needed:
+```bash
+ssh-keygen -t ed25519 -a 200 -C "[email protected]"
+```
+
 ### Example
 
 #### Executing remote ssh commands using password