Merge pull request #710 from tanzhongyibidu/master

add NOTICE.txt to address copyright

Disclaimer

+Apache brpc (incubating) is an effort undergoing incubation at The
+Apache Software Foundation (ASF), sponsored by the Apache Incubator PMC.
+
+Incubation is required of all newly accepted
+projects until a further review indicates that the
+infrastructure, communications, and decision making process have
+stabilized in a manner consistent with other successful ASF
+projects.
+
+While incubation status is not necessarily a reflection
+of the completeness or stability of the code, it does indicate
+that the project has yet to be fully endorsed by the ASF.
+

NOTICE

+Apache brpc (incubating)
+Copyright 2019 The Apache Software Foundation
+
+This product includes software developed at
+The Apache Software Foundation (http://www.apache.org/).
+
+Based on source code originally developed by
+Baidu (http://www.baidu.com/).