add an example with two separate compilation units

examples/f1.cc

@@ -0,0 +1,25 @@
+/*
+ * Make sure that the phmap.h header builds fine when included in two separate 
+ * source files
+ */
+#include <string>
+#include <parallel_hashmap/phmap.h>
+
+using phmap::flat_hash_map;
+ 
+int main()
+{
+    // Create an unordered_map of three strings (that map to strings)
+    using Map = flat_hash_map<std::string, std::string>;
+    Map email = 
+    {
+        { "tom",  "tom@gmail.com"},
+        { "jeff", "jk@gmail.com"},
+        { "jim",  "jimg@microsoft.com"}
+    };
+ 
+    extern void f2(Map&);
+    f2(email);
+ 
+    return 0;
+}

examples/f2.cc

@@ -0,0 +1,23 @@
+/*
+ * Make sure that the phmap.h header builds fine when included in two separate 
+ * source files
+ */
+#include <iostream>
+#include <string>
+#include <parallel_hashmap/phmap.h>
+
+using phmap::flat_hash_map;
+using Map = flat_hash_map<std::string, std::string>;
+
+void f2(Map& email)
+{
+    // Iterate and print keys and values 
+    for (const auto& n : email) 
+        std::cout << n.first << "'s email is: " << n.second << "\n";
+ 
+    // Add a new entry
+    email["bill"] = "bg@whatever.com";
+ 
+    // and print it
+    std::cout << "bill's email is: " << email["bill"] << "\n";
+}