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.

Also drop the config.json existence MUST because:

1. This section defines the configuration format, and doesn't need to
   be tied to a particular filename.
2. The bundle spec (in bundle.md) already has:

     This REQUIRED file MUST reside in the root of the bundle
     directory and MUST be named `config.json`.

The config.md line may have been useful when it was added (77d44b1,
Update runtime.md, 2015-07-16).  But since the bundle.md line landed
in 106ec2d (Cleanup bundle.md, 2015-10-02, #210), I think it's been
redundant.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

config.md

@@ -1,13 +1,12 @@
 # <a name="containerConfigurationFile" />Container Configuration file
 
-The container's top-level directory MUST contain a configuration file called `config.json`.
+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 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.