docs: rework readme

.github/CONTRIBUTING.md

@@ -0,0 +1,14 @@
+# Contributing 
+
+There are many ways to help out, here are some examples:
+
+* Help out with issues and PRs;
+* Contribute artwork under appropriate licenses (at least CC-BY-SA);
+* Discuss and be helpful to others on:
+  - IRC in the channel [#spex](https://web.libera.chat/#spex) on `irc.libera.chat`;
+  - [Zulip](https://spex.zulipchat.com/).
+* Contributed to releated projects:
+  - [`spexup`](https://github.com/spex-lang/spexup), the installer for the Spex
+    language;
+  - [`spex-lang.github.io`](https://github.com/spex-lang/spex-lang.github.io),
+    the Spex language website.