Add HttpReceiver as counterpart to the HttpSink (#49)

receivers/httpReceiver.md

@@ -0,0 +1,23 @@
+## `http` receiver
+
+The `http` receiver can be used receive metrics through HTTP POST requests.
+
+### Configuration structure
+
+```json
+{
+  "<name>": {
+    "type": "http",
+    "address" : "",
+    "port" : "8080",
+    "path" : "/write"
+  }
+}
+```
+
+- `type`: makes the receiver a `http` receiver
+- `address`: Listen address
+- `port`: Listen port
+- `path`: URL path for the write endpoint
+
+The HTTP endpoint listens to `http://<address>:<port>/<path>`