kernel version guide (#577)

* kernel version guide

* update

docs/kernel-versions.md

+# BPF Features by Linux Kernel Version
+
+Major milestone releases: 4.1, 4.4.
+
+## 3.18
+
+- bpf syscall.
+
+## 3.19
+
+- socket support: bpf can attach to sockets.
+
+## 4.1
+
+- kprobe support: BPF programs can now instrument any kernel function via kernel dynamic tracing.
+
+## 4.3
+
+- debug string support: bpf_trace_printk() supports strings.
+
+## 4.4
+
+- bpf_perf_event_output: used by many tools that print per-event output. Eg, opensnoop.
+- unprivileged BPF for sockets: non-root usage for socket-based programs.
+
+## 4.6
+
+- stack traces (BPF_MAP_TYPE_STACK_TRACE): for capturing stack traces as keys in maps. Eg, stackcount.
+
+## 4.7
+
+- tracepoint support (BPF_PROG_TYPE_TRACEPOINT): BPF programs can now use static kernel tracepoints.