config: Lead off with the purpose of the config

Instead of leading off with links to a bunch of other places, notes on
the Go tags, etc., make things more inviting by leading off with a
big-picture summary of what the configuration is about.

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

Author: wking

config.md

@@ -1,13 +1,13 @@
 # <a name="containerConfigurationFile" />Container Configuration file
 
+This configuration file contains metadata necessary to implement standard operations against the container.
+This includes the process to run, environment variables to inject, sandboxing features to use, etc.
+
 The container's top-level directory MUST contain a configuration file called `config.json`.
 The canonical schema is defined in this document, but there is a JSON Schema in [`schema/config-schema.json`](schema/config-schema.json) and Go bindings in [`specs-go/config.go`](specs-go/config.go).
 [Platform](spec.md#platforms)-specific configuration schema are defined in the [platform-specific documents](#platform-specific-configuration) linked below.
 For properties that are only defined for some [platforms](spec.md#platforms), the Go property has a `platform` tag listing those protocols (e.g. `platform:"linux,solaris"`).
 
-The configuration file contains metadata necessary to implement standard operations against the container.
-This includes the process to run, environment variables to inject, sandboxing features to use, etc.
-
 Below is a detailed description of each field defined in the configuration format and valid values are specified.
 Platform-specific fields are identified as such.
 For all platform-specific configuration values, the scope defined below in the [Platform-specific configuration](#platform-specific-configuration) section applies.