Update doc for sandbox and Github installs

The preferred (AKA the only way that fully works) way to
start the sanbox is via `bin/dev`. Same thing when installing
Solidus from Github or the local filesystem.

README.md

@@ -99,6 +99,15 @@ bundle add solidus
 bin/rails g solidus:install
 ```
 
+If you're installing Solidus from the `main` branch on GitHub or from your
+local filesystem, you'll need to build the new Solidus Admin Tailwind CSS assets
+and add the associated rake tasks to your application. To do this, you should run
+the following command instead:
+
+```bash
+bin/rails g solidus:install --build-admin-tailwind
+```
+
 And follow the prompt's instructions.
 ### Accessing Solidus Store
 
@@ -307,12 +316,17 @@ data already loaded.
     bin/sandbox
   ```
 
-* Start the server (`bin/rails` will forward any argument to the sandbox)
+* You can start the Rails server and other services from either the Solidus folder or the
+sandbox one by running the command:
 
   ```bash
-  bin/rails server
+  bin/dev
   ```
 
+Please note: if you run `bin/rails server` or similar commands, only the Rails server will
+start. This might cause the error `couldn't find file 'solidus_admin/tailwind.css'` when you
+try to load admin pages.
+
 ### Tests
 
 Solidus uses [RSpec](http://rspec.info) for tests. Refer to its documentation for