Support attaching to existing sqlite3* db instance.

Add intro in README.

README.md

@@ -4,7 +4,25 @@
 <a target="_blank" href="https://ci.appveyor.com/project/yangacer/sqlite3cpp">![Appveyor][badge.Appveyor]</a>
 <a target="_blank" href="https://coveralls.io/github/yangacer/sqlite3cpp?branch=master">![Coveralls][badge.Coveralls]</a>
 
-A C++17 wrapper library for the awsome sqlite3.
+Having a cool C/C++ function and want to use it with SQL of sqlite3?
+
+```cpp
+sqlite3 *db;
+sqlite3cpp::database mydb(db);
+mydb.create_scalar("coolFunc", [](std::string_view input) {
+  // do cool stuff
+  return "done!";
+});
+```
+That's it! Now you can use the `coolFunc` with the same database connection. e.g.
+
+```cpp
+char const *query = "select coolFunc(colA) from Table";
+sqlite3_exec(db, query, 0, 0, 0);
+// or
+mydb.execute(query);
+```
+
 
 ## Source
 

sqlite3cpp.cpp

@@ -159,6 +159,12 @@ database::database(std::string const &urn) {
   m_db.reset(i);
 }
 
+database::database(sqlite3 *db) : m_owned(false), m_db(db) {}
+
+database::~database() {
+  if (!m_owned) m_db.release();
+}
+
 cursor database::make_cursor() const noexcept { return cursor(*this); }
 
 std::string database::version() const { return SQLITE3CPP_VERSION_STRING; }

sqlite3cpp.h

@@ -213,6 +213,13 @@ struct SQLITE3CPP_EXPORT database {
   // filename. |urn| should be encoded in UTF-8.
   database(std::string const &urn);
 
+  // Attach to an opened sqlite3 database. Call site is responsible to mangage
+  // life time of the passed pointer |db|. sqlite3cpp does not release the
+  // pointer.
+  database(sqlite3* db);
+
+  ~database();
+
   // Create a cursor per current database for executing SQL statements.
   cursor make_cursor() const noexcept;
 
@@ -279,6 +286,7 @@ struct SQLITE3CPP_EXPORT database {
   static void final_ag(sqlite3_context *ctx);
   static void dispose_ag(void *user_data);
 
+  bool m_owned = true;
   std::unique_ptr<sqlite3, sqlite3_deleter> m_db;
 };