Add migration instructions to readme

README.md

@@ -16,3 +16,35 @@ When installing Ruby using `asdf install`, you can pass custom configure options
 
 * `RUBY_CONFIGURE_OPTIONS` - use only your configure options
 * `RUBY_EXTRA_CONFIGURE_OPTIONS` - append these configure options along with ones that this plugin already uses
+
+## Migrating from another Ruby version manager
+
+### `.ruby-version` file
+
+asdf uses the `.tool-versions` for auto-switching between software versions.
+To ease migration, you can you have it read an existing `.ruby-version` file to
+find out what version of Ruby should be used. To do this, add the following to
+`$HOME/.asdfrc`:
+
+    legacy_version_file = yes
+
+### Rubies
+
+asdf installs Rubies to `$HOME/.asdf/installs/ruby`, the directory structure is
+the same as other version managers, so you can just move the Rubies to this
+directory:
+
+#### RVM
+
+    mkdir ~/.asdf/installs/
+    mv ~/.rvm/rubies ~/.asdf/installs/ruby/
+
+#### rbenv
+
+    mkdir ~/.asdf/installs/
+    mv ~/.rbenv/versions ~/.asdf/installs/ruby/
+
+#### chruby
+
+    mkdir ~/.asdf/installs/
+    mv ~/.rubies ~/.asdf/installs/ruby/