Make install docs more clear

In the process of installing Exoframe myself, and was quite confused for a while until I found #141 (comment) which cleared it all up. Added a clarification to the server install docs.

Author: victorberland

docs/ServerInstallation.md

@@ -30,6 +30,7 @@ docker run -d \
 -v /home/user/.ssh/authorized_keys:/root/.ssh/authorized_keys:ro
 
 # this is your private key used for JWT encryption
+# you can set this to any string you want (a long random string generated by your password manager is recommended)
 -e EXO_PRIVATE_KEY=your_jwt_encryption_key
 
 # this is used to tell traefik to which deployment current docker service belongs