add license in README

README.md

@@ -291,3 +291,19 @@ See the [Reference Guide](docs/reference_guide.md) for more detail.
 <center><a href="images/bpftrace_internals_2018.png"><img src="images/bpftrace_internals_2018.png" border=0 width=700></a></center>
 
 bpftrace employs various techniques for efficiency, minimizing the instrumentation overhead. Summary statistics are stored in kernel BPF maps, which are asynchronously copied from kernel to user-space, only when needed. Other data, and asynchronous actions, are passed from kernel to user-space via the perf output buffer.
+
+## License
+
+Copyright 2019 Alastair Robertson
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.