glossary: Specify UTF-8 for all our JSON

I wish there was a cleaner reference for what UTF-8 was.  But [1]
seems too glib, and I can't find a more targetted link than just
dropping folks into a Unicode chapter (which is what [1] does):

  The Unicode Standard, Version 6.0, §3.9 D92, §3.10 D95 (2011)

With the current v8.0 (2015-06-17), it's still §3.9 D92 and §3.10 D95.

I'd rather put this normative requirement in the configuration-spec
files, but maintainer consensus was to put it in the glossary [2,3].

[1]: https://en.wikipedia.org/wiki/UTF-8
[2]: opencontainers#146 (comment)
[3]: opencontainers#146 (comment)

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

glossary.md

@@ -16,7 +16,14 @@ The [`config.json`](config.md) and [`runtime.json`](runtime-config.md) files in
 
 An environment setup for the [application](#application) (namespaces, resource limits, mounts, …).
 
+## JSON
+
+All configuration [JSON][] MUST be encoded in [UTF-8][].
+
 ## 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 an [application](#application) inside the container, and performs other [lifecycle actions](runtime.md).
+
+[JSON]: http://json.org/
+[UTF-8]: http://www.unicode.org/versions/Unicode8.0.0/ch03.pdf