Add some docs for editor

docs/l4d2_editor/editor_guide.md

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+The editor lets you move, rotate, color, an existing entity or create new ones from the prop spawner.
+
+# Modes
+
+There are 3 modes:
+
+- Move & Rotate
+
+    - The main mode, the entity will move to your cursor location, and can be rotated.
+
+    - Tools:
+
+        - Stacker Tool - Let's you automatically snap the next prop you spawn to the specified side of the previously spawned item. This lets you build a fence, automatically setting the next prop to the right
+
+        - Collision Rotation - Let's you have entities change their rotation to be off the normal vector of your cursor's location. This means when you looking at a wall, our a ceiling, the prop will act like that surface is the ground.
+
+- Color
+
+    - Lets you change the color of the entity
+
+- Scale (Wall Editor only)
+
+    - Let's you change the dimensions of the invisible wall
+
+- Free Look
+
+    - Lets you freely use your weapons and move around, to confirm the placement of your entity.
+
+# Controls
+
+The controls differ for each mode
+
+### Global (all modes)
+
+- Done / Spawn - `Use (E)`
+
+- Cancel - `Walk (SHIFT) + Use (E)`
+
+- Change Mode - `Zoom (MIDDLE MOUSE)`
+
+### Move & Rotate
+
+- Change spawn type (dynamic -> physics -> dynamic non-solid) - `Duck (CTRL) + Use (E)`
+
+- Rotate - Hold `Reload (R)`
+
+    - Move `mouse left/right` to change heading (or roll if axis changed)
+
+    - Move `mouse up/down` to change pitch
+
+    - While holding, `Left Mouse` to cycle axis from heading/pitch to roll
+
+    - While holding, `Right Mouse` to cycle through snap angles
+
+    - While holding, `Jump (SPACE)` to cycle the stacker tool
+
+    - While holding, `Walk (SHIFT)`, to toggle collision
+
+    - While holding, `Crouch (CTRL)`, to toggle collision rotate
+
+- `Left Mouse` to move entity farther away
+
+- `Right Mouse` to move the entity closer
+
+### Color
+
+Color components refer to the letter in RGBA (Red Green Blue Alpha)
+
+- Change color component (R G B A) - `Use (E)`
+
+- Decrease value of color component - `Left Mouse`
+
+- Increase value of color component - `Right Mouse`
+
+### Free Look
+
+No specific controls
+
+### Scale
+
+Only active when editing / creating invisible walls
+
+(broken)
+
+
+
+