docs: Document mirror mesh tool

readme.md

@@ -12,3 +12,22 @@ Usage:
 2. Select this tool.
 3. Pick a file to export to.
 4. Profit!
+
+## Mirror Mesh tool
+
+Used when you want to symmetrize a mesh across its X axis, but also wants its
+vertex weights correctly assigned after mirroring. This is different from the
+`Symmetrize` tool in stock Blender, which does not respect the symmetry of
+weight names and makes every weight group symmetric on itself.
+
+**This tool only mirrors from _Left (+X)_ to _Right (-X)_! It does not work in reverse!**
+
+Location: `Edit Mode > Mesh > Solstice Toolbox > Mirror Mesh`
+
+ID: `solstice.mirror_mesh`
+
+Usage:
+
+1. Go into edit mode.
+2. Select the tool.
+3. Profit!