1. git add submodule knowhere and set default branch

2. Update README.md about how to use git submodule


Former-commit-id: fd996485d8ef0b09055c4f3e8c20838ce9fd95ad

.gitmodules

+[submodule "cpp/thirdparty/knowhere"]
+	path = cpp/thirdparty/knowhere
+	url = git@192.168.1.105:xiaojun.lin/knowhere.git
+	branch = develop

cpp/README.md

@@ -12,6 +12,9 @@ cmake_build/src/milvus_server is the server
 
 cmake_build/src/libmilvus_engine.a is the static library
 
+    git submodule init
+    git submodule update
+    
     cd [sourcecode path]/cpp
     ./build.sh -t Debug
     ./build.sh -t Release

knowhere @ 291b3b42

+Subproject commit 291b3b422664f2509bab79d5cc63823dedbe903c