glossary: Provide a quick overview of important terms

And link them to the more detailed specification.

Subsection titles for the entries will be obnoxiously spacious, but
the other alternatives seem worse:

a. An HTML definition list (<dl>) would have nice default styling, but
   it's annoying to write raw HTML.  And we would have needed
   something like:

     <dt name="bundle">Bundle</dt>
     <dd>

     A [directory structure](bundle.md) that is...

     </dd>

   to get Markdown-style links in the defintion itself.

b. A Markdown list (* ...) would have reasonable default styling, but
   there's no Markdown syntax for adding anchors to the entries.  And
   a glossary is much less useful if you can't link to a specific
   entry.

Signed-off-by: W. Trevor King <wking@tremily.us>

README.md

@@ -15,6 +15,7 @@ Table of Contents
 - [Runtime and Lifecycle](runtime.md)
   - [Linux Specific Runtime](runtime-linux.md)
 - [Implementations](implementations.md)
+- [Glossary](glossary.md)
 
 # Use Cases
 

glossary.md

@@ -0,0 +1,18 @@
+# Glossary
+
+## Bundle
+
+A [directory structure](bundle.md) that is written ahead of time, distributed, and used to seed the runtime for creating a [container](#container) and launching a process within it.
+
+## Configuration
+
+The [`config.json`](config.md) and [`runtime.json`](runtime-config.md) files in a [bundle](#bundle) which define the intended [container](#container) and container process.
+
+## Container
+
+An environment for executing processes with configurable isolation and resource limitations (namespaces, resource limits, mounts, …).
+
+## Runtime
+
+An implementation of this specification.
+It reads the [configuration files](#configuration) from a [bundle](#bundle), uses that information to create a [container](#container), launches a process inside the container, and performs other [lifecycle actions](runtime.md).