Added information about a public key file in the SSH proxy section

Author: id3er0

README.md

@@ -120,6 +120,16 @@ As example, for Git:
  * `REMOTE_HOST_NAME` — name of a remote host, example: `git.example.com`;
  * `PRIVATE_KEY_FILE` — your private key file, example: `id_ed25519`.
 
+3. Add content of a public key file to the `root/.ssh/authorized_keys` file:
+
+ ```
+ cat PUBLIC_KEY_FILE >> root/.ssh/authorized_keys 
+ ```
+
+ Where:
+
+ * `PUBLIC_KEY_FILE` — your public key file, example `id_ed25519.pub`.
+
 ## Usage
 
 ### Start in background