lab4: Add README.md

lab4/README.md

+
+Pachetele primite/trimise de router vor avea urmatorul format:
+
+```
+typedef struct {
+    int len;
+    char payload[MAX_LEN];
+    int interface;
+} msg;
+```
+
+In implementarea laboratorului vom folosii urmatoarele functii: **get\_packet**
+care primeste un pachet si **send\_packet** care trimite un pachet pe o
+interfata specificata.
+
+```
+/* Blocking function, blocks if there is no packet to be received.
+On success returns 0 and populates m. m must be allocated! */
+int get_packet(msg *m);
+int send_packet(int interface, msg *m);
+```