Adds instructions for adding an icon (thanks @hristoterezov).

doc/adding-an-icon.md

+### Adding an icon to the font file (e.g. for the floating menu)
+1. Go to https://icomoon.io/app/
+2. Go to "Manage Projects" from the menu on the top left.
+3. Use "Import project" and select <code>fonts/selection.json</code> from Jitsi Meet.
+4. Import icons (e.g. svg files) using the "import items" button.
+5. Go to "generate font" and make sure the identifiers for the new icons are correct.
+6. Download the result in a zip file using the "download" button.
+7. Copy <code>selection.json</code> and <code>fonts/jitsi.*</code> from the zip file to <code>fonts/</code> in Jitsi Meet
+8. Copy the class for the new icon from <code>style.css</code> in the zip file to <code>css/font.css</code> in Jitsi Meet (do *not* copy the whole file)
+
+Sample commit: https://github.com/jitsi/jitsi-meet/commit/68bc819b89aec12364fcf07b81efa83a1900eed6
+