[hf] add docs for running 2 signal installations on 1 device

docs/2-signals-on-1-device.md

+# How to run 2 signal installations on one device
+
+Below is a guide for how to run a second installation of Signal on devices that already have an installation of signal running on them.
+
+This is useful if you would like to test signalboost using an throw-away account, or if you would like to interact with people over signal with a "YOLO account" that is not tied to your own identity and PII for work purposes.
+
+Below, we provide instructions for running 2 instances of Signal on both your phone and desktop. Currently, the guide assumes that you use an Android phone and run some debian-based distribution of Linux on your computer. We are happy to extend it to work for Apple devices!
+
+## Phone
+
+- on your android phone, set up a second User profile: Settings -> System -> Advanced -> Multiple Users -> Add user
+- switch to the new User's account
+- download signal to that User from here: https://signal.org/android/apk/
+- set up an account on this installation with a phone number from Twilio (using Twillio's "Incoming Messages" dash to retrieve the authentication code)
+
+## Computer
+
+In all of the below, we will assume that you have an alias for your backup account named "alice." Please substitute your actual alias username anywhere in the instructions below where you see "alice" or "Alice".
+
+### Create Data Directory and Link Device
+
+Create a new signal data directory:
+
+``` shell
+mkdir ~/.config/Signal-Alice
+```
+
+Run signal desktop from command line:
+
+``` shell
+signal-desktop --user-data-dir=$HOME/.config/Signal-Alice
+
+```
+
+Link this signal desktop to the new signal user you just set up on your phone (via "linked devices" flow in Signal mobile app's settings pane).
+
+### Create Desktop Entry
+
+If you want to startup your alternate signal client without using the command line, you can create a desktop entry. [1]
+
+Create a new desktop entry file:
+
+``` shell
+sudo touch /usr/share/applications/signal-desktop-alice
+```
+
+Edit that file to have the following contents
+
+```
+[Desktop Entry]
+Name=Signal-Alice
+Exec=/opt/Signal/signal-desktop --no-sandbox --user-data-dir=$HOME/.config/Signal-Alice %U
+Terminal=false
+Type=Application
+Icon=signal-desktop
+StartupWMClass=Signal-Alice
+Comment=Private messaging from your desktop
+MimeType=x-scheme-handler/sgnl;x-scheme-handler/signalcaptcha;
+Categories=Network;InstantMessaging;Chat;
+```
+
+Now, you should be able to start your alias instance of signal desktop by typing `Signal-Alice` into the GNOME App Launcher (or equivalent).
+
+[1] see here for a generic guide on desktop entries: https://commithub.com/how-to-create-gnome-desktop-entry