eBPF.md: Document atomic add instructions

Atomic add instructions have been available since the early days of
eBPF, although BPF_XADD has later been renamed into BPF_ATOMIC. Comes in
32 and 64 bits flavours.

This is supported in the kernel since commit:
bd4cf0ed331a ("net: filter: rework/optimize internal BPF interpreter's instruction set")

Author: qmonnet

eBPF.md

@@ -225,3 +225,17 @@ Opcode | Mnemonic            | Pseudocode
 0xce   | jslt dst, src, +off | PC += off if dst < src (signed)
 0xd6   | jsle dst, imm, +off | PC += off if dst <= imm (signed)
 0xde   | jsle dst, src, +off | PC += off if dst <= src (signed)
+
+## Atomic Instructions
+
+### 64-bit
+
+Opcode             | Mnemonic                   | Pseudocode
+-------------------|----------------------------|---------------------------------------------------------
+0xdb (imm == 0x00) | add [dst+off], src         | (dst + off) += src
+
+### 32-bit
+
+Opcode             | Mnemonic                   | Pseudocode (uint32_t * casts omitted for readability)
+-------|----------------------------|---------------------------------------------------------------------
+0xc3 (imm == 0x00) | add32 [dst+off], src       | (dst + off) += src