added submodule phmap

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "parallel-hashmap"]
+	path = parallel-hashmap
+	url = https://github.com/greg7mdp/parallel-hashmap

include/user.hpp

@@ -8,6 +8,7 @@ private:
     uint_fast64_t balance;
     std::string password;
 
+    //for read/write of object's state concurrently
     std::mutex bal_lock;
     std::mutex pass_lock;
 

parallel-hashmap

@@ -0,0 +1 @@
+Subproject commit 50558ca3f71c2a19f839f4fa26ad034470f4f3c6