example/tailcall: add README (#133)

example/tailcall_minimal/README.md

@@ -0,0 +1,30 @@
+# tailcall example
+
+This is an example demonstrating userspace to kernel tailcall.
+
+## Usage
+
+```
+make -j$(nproc)
+```
+
+### Terminal 1
+```
+bpftime load ./tailcall_minimal
+```
+
+### Terminal 2
+
+```
+bpftime start ./victim
+```
+
+### Behavior
+
+See `/sys/kernel/debug/tracing/trace_pipe ` and check whether there are lines containing `Invoked!`
+
+## About this example
+
+`tailcall_minimal.bpf.c` itself is a uprobe ebpf program. It will be triggered when `./victim:add_func` was invoked, and it will run `bpf_tail_call` to call a kernel ebpf program, whose kernel fd is stored in the prog array.
+
+`tailcall_minimal.c` is a loader program. It executes the original syscall, loads an ebpf program into kernel (which printf `Invoked!` to trace_pipe), and stores kernel fd into the userspace prog array