There is a little trick to adding an email signature and preserving your email formatting on an iOS device!

The first step is to email yourself from your computer so that you have the proper formatting for your signature like so.

Then once you have done that, open up said email on your iOS device. Once there, copy and paste the signature by holding down your finger and hitting Select.

Once you have done that, your screen should look like this.

Now drag the left pin to highlight the entire signature so that it looks like this.

Then hit copy!

Now go to Mail, Contacts, Calendars –> Signatures and either select All Accounts or Per Account depending on how you want the signature applied.

Be sure and erase the old signature, then press and hold your finger to the screen until these options come up. Now press Paste.

Finally, physically shake your iPhone until this screen comes up

and click Undo. Once you click that, your signature will contain the proper formatting.