Explain origin of these files.

Author: MarkCallow

external/lodepng/README.md

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+# LodePNG
+
+Sourced from https://github.com/KhronosGroup/lodepng/tree/split_decode_inflate which is a branch in a fork of https://github.com/lvandeve/lodepng.
+
+The branch in the fork has been modified to split decode and data inflation to allow it to work with our imageio library. A PR will be submitted to merge these changes into https://github.com/lvandeve/lodepng.
+
+The repo is 3MB but only the 2 files here are needed and it has no tags so 
+using `git submodule` or `git subrepo` are unattractive options.
+
+