Add "logo"

I chose to create an image with the Rust logo plus something representing Objective-C; since they don't have a logo, I used the short form "Obj-C" enclosed in square brackets which is kind of a play on the message sending syntax in Objective-C. I've seen this style elsewhere, so no invention on my part.

The Rust logo is borrowed from here:
https://github.com/rust-lang/rust-artwork/blob/809592a8fafee0c70279d80905aba768aa1dd035/logo/rust-logo-blk.svg

Since this crate is not commercial, and is clearly related to (but not officially part of) the Rust project, usage of the trademark should be acceptable.

README.md

@@ -1,4 +1,4 @@
-# Objective-C in Rust
+# [![Rust + \[Obj-C\]](assets/logo-small.png)](https://github.com/madsmtm/objc2) Objective-C in Rust
 
 [![License](https://badgen.net/badge/license/MIT/blue)](../LICENSE.txt)
 [![CI Status](https://github.com/madsmtm/objc2/workflows/CI/badge.svg)](https://github.com/madsmtm/objc2/actions)