openssl key and csr generation commands

[konklone]

Add these to the README and put them in context:

# generate an encrypted private 2048-bit key
openssl genrsa -aes256 -out my-private-encrypted.key 2048

# decrypt the key, in order to generate the CSR
openssl rsa -in my-private-encrypted.key -out my-private-decrypted.key

# generate the CSR (delete decrypted key unless this is the production server)
openssl req -new -sha256 -key my-private-decrypted.key -out mydomain.com.csr

[konklone]

Addressed by #9.