Introduce railsrc (#728)

Introduces [railsrc][1] in an effort to prepare for the eventual
[removal of the suspenders executable][2].

With this change, calling `rails new` will automatically set the
following options:

```
--database=postgresql
```

If you need to use another database, you can override this value like
so: `rails new my_app --database=mysql` or `rails new my_app --no_rc` to
skip this file all together

[1]: https://github.com/rails/rails/blob/7f7f9df8641e35a076fe26bd097f6a1b22cb4e2d/railties/lib/rails/generators/rails/app/USAGE#L5C1-L7
[2]: https://github.com/thoughtbot/suspenders/blob/main/GOALS.md#from-here-to-there

README.md

@@ -214,6 +214,14 @@ configuration:
 - Add trusted binstubs to the `PATH`.
 - Load the ASDF version manager.
 
+[Rails](https://rubyonrails.org)
+
+- Adds [railsrc][] with `--database=postgresql`. If you need to use another
+  database, you can override this value like so: `rails new my_app --database=mysql` or
+`rails new my_app --no_rc`
+
+[railsrc]: https://github.com/rails/rails/blob/7f7f9df8641e35a076fe26bd097f6a1b22cb4e2d/railties/lib/rails/generators/rails/app/USAGE#L5C1-L7
+
 Shell aliases and scripts:
 
 - `b` for `bundle`.

railsrc

@@ -0,0 +1 @@
+--database=postgresql