Add description for how to configure ASDF (#90)

Author: russell

README.md

@@ -20,6 +20,8 @@ We recently switched from the base branch of `develop` to the base of `master`.
   - [`BUILD.bazel` file(s)](#buildbazel-files)
     - [Define Ruby Executable, Library and an RSpec](#define-ruby-executable-library-and-an-rspec)
     - [Package Ruby files as a Gem](#package-ruby-files-as-a-gem)
+  - [Tool Specific Setup](#tool-specific-setup)
+    - [ASDF](#asdf)
   - [Rule Dependency Diagram](#rule-dependency-diagram)
 - [Rules](#rules)
   - [`ruby_library`](#ruby_library)
@@ -226,6 +228,18 @@ ruby_gem(
 
 ```
 
+### Tool Specific Setup
+
+#### ASDF
+
+If you are using ASDF to manage your ruby installs, you can use them by adding `.bazelrc`:
+
+```
+build --test_env=ASDF_DIR --test_env=ASDF_DATA_DIR
+build --action_env=ASDF_DIR --test_env=ASDF_DATA_DIR
+```
+You will have to be sure to export the `ASDF_DATA_DIR` in your profile since it's not set by default. e.g. `export ASDF_DATA_DIR="$HOME/.asdf"` 
+
 ### Rule Dependency Diagram
 
 > NOTE: this diagram is slightly outdated.