docs: add userspace communication to readme

README.md

@@ -20,3 +20,28 @@ The binaries will be in:
 - `build/daemon/daemon`
 
 - `build/clients/*`
+
+## Communication
+
+### Userspace
+
+The client programs that aggregate the desired system data may send them to the daemon process.
+This is done via a simple packet, `daemon_measurement_t` which is defined in [`common/include/protocol.h`](common/include/protocol.h):
+```c
+#define MAX_DAEMON_PACKET_SIZE PIPE_BUF
+#define DAEMON_PACKET_HEADER_SIZE sizeof(u_int8_t) + sizeof(u_int16_t)
+#define MAX_DAEMON_PACKET_DATA_SIZE MAX_DAEMON_PACKET_SIZE - DAEMON_PACKET_HEADER_SIZE
+
+typedef struct {
+    uint8_t measurement_id;
+    uint16_t size;
+    uint8_t data[MAX_DAEMON_PACKET_DATA_SIZE];
+} daemon_measurement_t;
+```
+
+The packet size equals to the size of `PIPE_BUF` (4096).
+The communication to the deamon is implemented via a named pipe, the size ensure that the packet is atomic, i.e. it is written to the pipe in one go.
+
+- The first byte contains the measurement id, so the maximum number of different measurements client is 255.
+- The second and third bytes contain the size of the data in the packet.
+- All following bytes contain the data.