Ubuntu support and mise install guidance

Author: jeremy

README.md

@@ -1,17 +1,26 @@
-## Old Rubies, meet new macOS
+## Old Rubies, meet new macOS and Ubuntu
 
-Trying to run an old, unsupported Ruby on an M1 Mac? You're in the right place.
+Trying to run an old, unsupported Ruby? You're in the right place.
 
-This is a collection of ruby-build definitions for local dev on macOS. Not for production use. Homebrew is required.
+This is a collection of ruby-build definitions for local dev on macOS and Ubuntu. Not for production use. macOS (and Homebrew) or Ubuntu are required.
 
-Ruby versions:
-* 1.8.7-p374
+### mise
 
+```bash
+git clone https://github.com/basecamp/ruby-dev
 
-### How to install
+RUBY_BUILD_DEFINITIONS="~/path/to/ruby-dev" mise use [email protected]
+```
 
-Grab the build definition by cloning the repo or `curl`ing a specific file, then `rbenv install` the path to the definition.
+### rbenv
 
+```bash
+git clone https://github.com/basecamp/ruby-dev
+
+RUBY_BUILD_DEFINITIONS="./ruby-dev" rbenv install 1.8.7-p374
+```
+
+or
 ```bash
 git clone https://github.com/basecamp/ruby-dev
 cd ruby-dev