2023-06-28 Add section on setting up multiple hosts

Issue #382 suggests adding a section explaining how to use the same
YubiKey on multiple hosts.

This PR incorporates most of the suggestions.

Fixes: #382

Signed-off-by: Phill Kelley <34226495+Paraphraser@users.noreply.github.com>

README.md

@@ -48,6 +48,7 @@ If you have a comment or suggestion, please open an [Issue](https://github.com/d
 - [Verify card](#verify-card)
 - [Multiple YubiKeys](#multiple-yubikeys)
   - [Switching between two or more Yubikeys](#switching-between-two-or-more-yubikeys)
+- [Multiple Hosts](#multiple-hosts)
 - [Cleanup](#cleanup)
 - [Using keys](#using-keys)
 - [Rotating keys](#rotating-keys)
@@ -1677,6 +1678,79 @@ To return to using the second Yubikey just repeat (insert other Yubikey and re-r
 	
 Obviously this command is not easy to remember so it is recommended to either create a script or a shell alias to make this more user friendly.
 
+# Multiple Hosts
+
+It can be convenient to use your YubiKey on multiple hosts:
+
+* a desktop plus a laptop
+* home and work computers
+* an environment like [Tails](https://tails.boum.org)
+
+The simplest way to set up a second host is to begin by exporting your public key and trust settings on the host where your YubiKey is already working:
+
+``` console
+$ gpg --armor --export $KEYID > gpg-public-key-$KEYID.asc
+$ gpg --export-ownertrust > gpg-owner-trust.txt
+```
+
+Move both files to the second host. Then, on the second host:
+
+1. Define your KEYID. For example:
+
+	``` console
+	$ export KEYID=0xFF3E7D88647EBCDB
+	``` 
+
+2. Import your public key:
+
+	``` console
+	$ gpg --import gpg-public-key-$KEYID.asc
+	```
+
+3. Import the trust settings:
+
+	``` console
+	$ gpg --import-ownertrust < gpg-owner-trust.txt
+	```
+
+4. Insert your YubiKey into a USB port.
+5. Import the private key stubs from the YubiKey:
+
+	``` console
+	$ gpg --card-status
+	```
+
+If you need to set up a second host when you are travelling and don't have ready access to your primary host, you can import your public key from a key-server and set trust manually:
+
+1. Define your KEYID. For example:
+
+	``` console
+	$ export KEYID=0xFF3E7D88647EBCDB
+	``` 
+
+2. Fetch the public key from a key-server. For example:
+
+	``` console
+	$ gpg --keyserver hkps://keyserver.ubuntu.com:443 --recv $KEYID
+	```
+	
+3. Set ultimate trust:
+
+	``` console
+	$ gpg --edit-key $KEYID
+	gpg> trust
+	Your decision? 5
+	Do you really want to set this key to ultimate trust? (y/N) y
+	gpg> quit
+	```
+
+4. Insert your YubiKey into a USB port.
+5. Import the private key stubs from the YubiKey:
+
+	``` console
+	$ gpg --card-status
+	```
+	
 # Cleanup
 
 Before finishing the setup, ensure you have done the following: