Added gfxreconstruct readme.

gfxreconstruct/README.md

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+# gfxreconstruct
+
+Steam Frame comes pre-loaded with the gfxreconstruct Vulkan layer.
+
+To enable when launching the game from Steam:
+```
+nano ~/.config/steam/env/gfx.txt
+```
+Add the following environment variable settings:
+```
+VK_INSTANCE_LAYERS=VK_LAYER_LUNARG_gfxreconstruct
+GFXRECON_CAPTURE_FILE=/tmp/gfxrecon_capture.gfxr
+GFXRECON_CAPTURE_FILE_FLUSH=1
+DXVK_DEBUG=markers
+```
+Once you reach the point you want to capture, exit and find your trace(s) in /tmp/gfxrecon*.gfxr
+
+If there are multiple, you probably want the biggest one from that capture.
+
+Don't forget to comment out or remove the lines from gfx.txt when you're done capturing.
+
+See https://github.com/LunarG/gfxreconstruct for more details.