Add multisite properties documentation. (#1124)

readme/multisite.md

@@ -41,3 +41,18 @@ The default Drush site aliases provided by [Acquia Cloud](https://docs.acquia.co
 It's recommended to copy the aliases file provided by Acquia Cloud or Club to create a separate aliases file for each site. Simply modify the `uri` and `parent` keys for the aliases within each file to match the correct database / site.
 
 TODO: Add instructions for integration with BLT development workflows and DrupalVM.
+
+## Multisite tasks
+
+You may override BLT variables on a per-site basis by creating a `site.yml` file in `docroot/sites/[site-name]/`. You may then run BLT with the `multisite.name` variable set at the command line to load the site's properties. 
+
+For instance, if the `drush` aliases for your site in `docroot/sites/mysite` were `@mysite.local` and `@mysite.test`, you could define these in `docroot/sites/mysite/site.yml` as:
+
+```yaml
+drush:
+  aliases:
+    local: mysite.local
+    remote: mysite.test
+```
+
+Then, to refresh your local site, you could run: `blt local:refresh -Dmultisite.name=mysite`.