Add provenance query param documentation

Describe the effects (and provide example output) of
the new provenance query parameter.

Signed-off-by: Peter W. Morreale <pmorreale@statestreet.com>

docs/content/rest-api.md

@@ -725,6 +725,7 @@ The path separator is used to access values inside object and array documents. I
 
 - **input** - Provide an input document. Format is a JSON value that will be used as the value for the input document.
 - **pretty** - If parameter is `true`, response will formatted for humans.
+- **provenance** - If parameter is `true`, response will include build/version info in addition to the result.  See [Provenance](#provenance) for more detail.
 - **explain** - Return query explanation in addition to result. Values: **full**.
 - **metrics** - Return query performance metrics in addition to result. See [Performance Metrics](#performance-metrics) for more detail.
 - **instrument** - Instrument query evaluation and return a superset of performance metrics in addition to result. See [Performance Metrics](#performance-metrics) for more detail.
@@ -1042,6 +1043,7 @@ The request body contains an object that specifies a value for [The input Docume
 
 - **partial** - Use the partial evaluation (optimization) when evaluating the query.
 - **pretty** - If parameter is `true`, response will formatted for humans.
+- **provenance** - If parameter is `true`, response will include build/version info in addition to the result.  See [Provenance](#provenance) for more detail.
 - **explain** - Return query explanation in addition to result. Values: **full**.
 - **metrics** - Return query performance metrics in addition to result. See [Performance Metrics](#performance-metrics) for more detail.
 - **instrument** - Instrument query evaluation and return a superset of performance metrics in addition to result. See [Performance Metrics](#performance-metrics) for more detail.
@@ -1935,6 +1937,51 @@ Query instrumentation can help diagnose performance problems, however, it can
 add significant overhead to query evaluation. We recommend leaving query
 instrumentation off unless you are debugging a performance problem.
 
+## Provenance
+
+OPA can report provenance information at runtime. Provenance information can
+be requested on individual API calls and are returned inline with the API
+response. To obtain provenance information on an API call, specify the
+`provenance=true` query parameter when executing the API call. Provenance information
+is currently supported for the following APIs:
+
+- Data API (GET and POST)
+
+For example:
+
+```http
+POST /v1/data/example?provenance=true HTTP/1.1
+```
+
+Response:
+
+```http
+HTTP/1.1 200 OK
+Content-Type: application/json
+```
+
+```json
+{
+  "provenance": {
+    "build_commit": "1955fc4d",
+    "build_host": "foo.com",
+    "build_timestamp": "2019-04-29T23:42:04Z",
+    "revision": "ID-b1298a6c-6ad8-11e9-a26f-d38b5ceadad5",
+    "version": "0.10.8-dev"
+  },
+  "result": true
+}
+```
+
+OPA currently supports the following query provenance information:
+
+- **version**: The version of this OPA instance.
+- **build_commit**: The git commit id of this OPA build.
+- **build_timestamp**: The timestamp when this instance was built.
+- **build_host**: The hostname where this instance was built.
+- **revision**: The _revision_ string included in a .manifest file (if present) within a bundle.
+
+
 ## Watches
 
 OPA can set watches on queries and report whenever the result of evaluating the query has changed. When a watch is set on a query, the requesting connection will be maintained as the query results are streamed back in HTTP Chunked Encoding format. A notification reflecting a certain change to the query results will be delivered _at most once_. That is, if a watch is set on `data.x`, and then multiple writes are made to `data.x`, say `1`, `2` and `3`, only the notification reflecting `data.x=3` is always seen eventually (assuming the watch is not ended, there are no connection problems, etc). The notifications reflecting `data.x=1` and `data.x=2` _might_ be seen. However, the notifications sent are guaranteed to be in order (`data.x=2` will always come after `data.x=1`, if it comes).