feat: Adds the rust_icu mascot

It's the drawing of Quintus Junius Rusticus, as shown in Crabb's
historical dictionary.

The drawing miraculously combines multiple allusions to ICU, Rust and
crustaceans in one place. This, combined with the Breugelian rendition,
makes it an ideal mascot for rust_icu project.

README.md

@@ -8,6 +8,14 @@ README         | https://github.com/google/rust_icu/blob/main/README.md
 Coverage       | [View report](/coverage/report.md)
 Docs           | https://docs.rs/crate/rust_icu
 
+![Quintus Junius Rusticus from Crabb's Historical Dictionary (lived c. 100-c. 170 AD), consul in 133 (consul suffectus) and 162 (consul ordinarius) - https://commons.wikimedia.org/wiki/File:Quintus_Junius_Rusticus_from_Crabb%27s_Historical_Dictionary.jpg#filelinks](docs/204px-Quintus_Junius_Rusticus_from_Crabbs_Historical_Dictionary.jpg)
+
+*Project mascot: Quintus Junius Rusticus from Crabb's Historical Dictionary
+(lived c. 100-c. 170 AD), consul in 133 (consul suffectus) and 162 (consul
+ordinarius). Source: [Wikimedia Commons][wmc]*
+
+[wmc]: https://commons.wikimedia.org/wiki/File:Quintus_Junius_Rusticus_from_Crabb%27s_Historical_Dictionary.jpg#filelinks
+
 This is a library of low level native rust language bindings for the
 International Components for Unicode (ICU) library for C (a.k.a. ICU4C).