Add List All Installable Rails Versions as a Rails til

rails/list-all-installable-rails-versions.md

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+# List All Installable Rails Versions
+
+_Here's a [screencast](https://www.youtube.com/watch?v=IizkvqGLkhU) in case you
+want to watch instead of read._
+
+I [was curious](https://twitter.com/jbrancha/status/1345467867479875584?s=20)
+what versions of Rails were remotely available to be installed with `gem`. On
+its own, `gem list rails` will show all _locally_ installed gems that
+partially match `rails`. That'll include gems like `rspec-rails` and
+`sprockets-rails`.
+
+First, the `--exact` flag can be added to narrow down the results to an exact
+match of `rails`.
+
+Then, the `--remote` flag can be included to request `gem` look for remote
+versions. That is, versions available on the rubygems server.
+
+Lastly, the `--all` flag can be included to fetch all versions instead of only
+the latest version.
+
+Putting it all together:
+
+```bash
+$ gem list rails --exact --remote --all
+rails (6.1.0, 6.0.3.4, 6.0.3.3, ... 0.9.0, 0.8.5, 0.8.0)
+```
+
+[source](https://stackoverflow.com/a/9146057/535590)