Create README.md for December 15, 2025 assets

Added guidelines for asset filenames and formats for the December 15, 2025 newsletter.

Author: TheKitty

assets/20251215/README.md

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+## December 15, 2025 Python on Microcontrollers Newsletter Assets
+
+Please preface all asset filenames for this issue with 20251215, re. 20251215neopixel.jpg
+
+Please consider using JPG files for size. Use a .jpg file extension, not .JPG or .jfif, please.
+.png files are acceptable too. Please, not .PNG or .svg files.
+
+* All images **must be <=550px wide**
+* Please avoid letterboxed images, e.g. images with black bars.
+
+For videos:
+
+* Capture via https://twittervideodownloader.com/ if necessary
+* Convert to GIF with https://ezgif.com/, max 550 px wide, **file MUST BE < 1MB in size**
+
+To reduce GIF filesize, make them smaller or crop, say to <400px wide, and consider 5 seconds max.