Update themes.md

Author: MichaelCurrin

recipes/jekyll/themes.md

@@ -1,6 +1,30 @@
 # Themes
 > Creating and maintaining a GitHub theme
 
+## Notes
+
+See the docs for structure in general not just Jekyll - [What is a gem?](https://guides.rubygems.org/what-is-a-gem/).
+
+See [Specification](https://ruby-doc.org/stdlib-2.7.0/libdoc/rubygems/rdoc/Gem/Specification.html) in Ruby docs.
+
+### Gemspec
+
+- The `*.gemspec` file is used to package the theme as gem. 
+- Here are ways to add required dependencies. These are not actually installed when you install the gem, but the theme will give an error to say they are missing (at least this is the way it works with Remote Theme.
+    ```ruby
+    spec.add_runtime_dependency 'example'
+    spec.add_runtime_dependency 'example', '~> 1.1', '>= 1.1.4'
+    
+    spec.add_dependency 'example' # alias for add_runtime_dependency
+    
+    spec.add_development_dependency 'example'
+    ```
+
+### Gemfile
+
+- A `Gemfile` is not needed in the theme. If it is included, it is only for testing the project and it is not passed to a downstream project.
+- The `gemspec` command in the `Gemfile` will install dependencies from the `*.gemspec` file.
+
 
 ## Samples
 
@@ -10,12 +34,18 @@ My own quickstart theme project.
 
 [![MichaelCurrin - jekyll-theme-quickstart](https://img.shields.io/static/v1?label=MichaelCurrin&message=jekyll-theme-quickstart&color=blue&logo=github)](https://github.com/MichaelCurrin/jekyll-theme-quickstart)
 
+### Installed
+
+Have a look at gemspec files for installed gems in your project:
+
+```sh
+$ ls vendor/bundle/ruby/2.7.0/specifications
+```
+
 ### Minima
 
 Pieces from the [Minima](https://github.com/jekyll/minima) theme, to help with build your theme.
 
-As in Minima, you can add gems to your theme's `*.gemspec` file. That means you don't have to add them to your project's `Gemfile`.
-
 - `Gemfile`
     ```ruby
     # frozen_string_literal: true