Add permanent images (#1878)

Co-authored-by: gillespi314 <73313222+gillespi314@users.noreply.github.com>

assets/images/permanent/README.md

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+# images/permanent/
+
+Permanent static images
+
+These images are made available in this particular folder to guarantee that their URLs will be accessible on the internet forever.  (Or as close to that as we can.)
+We'll be careful to never change these URLs, lest we break images for folks.
+
+> Why put these in a separate folder?
+> Just to make it harder to accidentally break the URLs by moving these images around, deleting them, or renaming them.
+> If we want to deprecate one of these images, set up an appropriate redirect for it in config/routes.js.
+
+
+## Adding an image
+
+To add an image, simply add an image to this folder using Fleet's standard image naming convention:
+
+For example:
+```
+images/permanent/icon-avatar-default-128x128@2x.png
+```
+
+Then, after merging, your image will be available from anywhere that can talk to the public internet!
+
+For example:
+```
+<img alt="An unbreakable image of a lovely default avatar." src="https://fleetdm.com/images/permanent/icon-avatar-default-128x128@2x.png"/>
+```