doc: include RNG documentation

docs/protocol/data-requests/rng.md

@@ -0,0 +1,52 @@
+# Random Number Generation
+
+## Witnet RNG request
+The Witnet RNG request shares the same structure as a standard request with the 3 stages (retrieval, aggregation and 
+tally) and it will be solved by the same witnesses that are currently working in the Witnet mainnet.
+
+### Retrieval
+As a source for RNG requests, we specify a new `RADType` called `Rng`. This new source will trigger the creation of a
+random vector of 32 bytes, instead of reading data from an API source. The rest of fields are unnecessary: the `url` 
+field must be empty and the `script` field will be ignored, so we can specify the CBOR code for an empty array 
+(`[128]`). A single source is used in the retrieval section of this kind of requests.
+
+```rust
+"retrieve":[
+    {
+        "kind":"RNG",
+        "url":"",
+        "script":[128]
+    }
+]
+```
+
+### Aggregation
+Aggregation is not necessary due to the fact that RNG requests have only one retrieval. To unwrap the `RadonArray` 
+into a single element, we will use the `Mode` as a `RadonReducer` without filters.
+
+```rust
+"aggregate":{
+    "filters": [], 
+    "reducer": 2,
+}
+```
+
+### Tally
+For the tally stage, a new `RadonReducer` is required, `HashConcatenate`. This reducer takes the `RadonBytes` revealed 
+by the witnesses, concatenates them into a single vector of bytes and hash them with SHA256. In the same way, as in the 
+aggregation stage, no filters are required.
+
+```rust
+"tally":{
+    "filters": [], 
+    "reducer": 11,
+}
+```
+
+## Conclusions
+The random numbers (actually, random bytes) generated by the Witnet protocol are the result of applying a hash 
+function on a crowdsourced seed coming from the random bytes provided by several different witnesses, as selected 
+randomly by the Witnet protocol. That is, the randomness property of these is powered by the decentralized nature 
+of the Witnet network.
+
+

mkdocs.yml

@@ -49,6 +49,7 @@ nav:
       - Witnet Requests:
           - Overview: protocol/data-requests/overview.md
           - Encoding: protocol/data-requests/radon-encoding.md
+          - RNG:  protocol/data-requests/rng.md
       #- protocol/serialization.md
   #- Operator Docs:
       #- Run an Ethereum bridge: node-operators/ethereum-bridge.md