Add icu4x project as user to README (#1593)

* add icu4x user to README

* reduce resolution of icu4x combination mark

* increase resolution to 200pixels width

The icu4x logo with just 120pixels width looked noticibly shabby even in the README, so we increase to 200pixels width.

* make height=64 pixels

This works best for Wasmi's readme as it shrinks logos to height=32. While height=32 source looks very bad, the same image with twice the height (=64) looks very good while being almost the same size as the previous 200pixels width.

Author: Robbepop

README.md

@@ -175,6 +175,12 @@ If you want your project on this list [please inform me](mailto:robin.freyler@gm
     height="32" style="vertical-align: middle;" alt="Munal OS"
   />
 </a>&nbsp;
+<a href="https://github.com/unicode-org/icu4x">
+  <img
+    src="./resources/logos/users/icu4x.png"
+    height="32" style="vertical-align: middle;" alt="icu4x"
+  />
+</a>&nbsp;
 <a href="https://github.com/Uni-Gal/Ayaka">
   <img
     src="./resources/logos/users/ayaka.png"