Update README.

Author: gamemann

README.md

@@ -11,6 +11,11 @@
 * UDP, TCP, and ICMP layer 4 protocols supported.
 * Optional layer 3 and 4 checksum calculation in the event you want the NIC's hardware to calculate checksums for generated outgoing packets.
 
+#### Note Regarding Seeding/Randomness
+When generating randomness in a packet (e.g. random source IPs, ports, payload(s), etc.), by default, we try to use nanoseconds since boot using `timespec` as the seed when passing to `[rand_r()]`(https://linux.die.net/man/3/rand_r) for example. However, I've found this isn't completely random, especially since the seed variable (`unsigned int`) and `timespec` nanoseconds variable (`long`) do not match in size.
+
+Therefore, I added the constant [`VERY_RANDOM`](https://github.com/Packet-Batch/PB-AF-XDP/blob/master/src/sequence.h#L36) that can be uncommented. When this constant is defined, the `[getrandom()]`(https://man7.org/linux/man-pages/man2/getrandom.2.html) function is used to fill the seed with random bytes. This results in more randomness, but the `getrandom()` function adds a bit more overhead than retrieving the nanoseconds since system boot from my testing.
+
 ### Disclaimer
 I do **NOT** support using these tools maliciously or as a part of a targeted attack. I've made these tools to perform penetration tests against my own firewalls along with occasionally debugging network issues such as packets not arriving to their destination correctly.