Docs: Empty captures array implies start sample of 0 (#327)

* Empty captures array implies start sample of 0

* Clarify `captures` object description in schema

sigmf-schema.json

@@ -158,7 +158,7 @@
       "additionalProperties": true
     },
     "captures": {
-      "description": "The `captures` Object is an array of capture segment objects that describe the parameters of the signal capture. It MUST be sorted by the value of each capture segment's `core:sample_start` key, ascending.  Capture Segment Objects are composed of key/value pairs, and each Segment describes a chunk of samples that can be mapped into memory for processing. Each Segment MUST contain a `core:sample_start` key/value pair, which indicates the sample index relative to the Dataset where this Segment's metadata applies. The fields that are described within a Capture Segment are scoped to that Segment only and need to be explicitly declared again if they are valid in subsequent Segments. ",
+      "description": "The `captures` Object is an array of capture segment objects that describe the parameters of the signal capture. It MUST be sorted by the value of each capture segment's `core:sample_start` key, ascending.  Capture Segment Objects are composed of key/value pairs, and each Segment describes a chunk of samples that can be mapped into memory for processing. Each Segment MUST contain a `core:sample_start` key/value pair, which indicates the sample index relative to the Dataset where this Segment's metadata applies. The fields that are described within a Capture Segment are scoped to that Segment only and need to be explicitly declared again if they are valid in subsequent Segments. While it is recommended there be at least one segment defined, if there are no items in the captures array it is implied that a single capture exists with `core:sample_start` equal to zero (no other metadata is implied), i.e., `\"captures\": []` implies `\"captures\": [{\"core:sample_start\": 0}]`.",
       "default": [],
       "type": "array",
       "additionalItems": false,