Update README.md

README.md

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 # QtBigInt
+
+QtBigInt - Arbitrary-sized integer class for C++ and build system of Qt (qmake)
+
+
+# Features
+* No additional dependencies other than the standard library.
+* Support for all arithmetic operators, which allows you to easily integrate the library into any of your algorithms.
+* Based on the minigmp 6.1.2 library that provides high performance.
+
+# Build
+
+
+ * git clone https://github.com/QuasarApp/QtBigInt.git
+ * cd QtBigInt
+ * qmake -r 
+ * make -j8
+ * make test #(for testing)
+
+# Include
+
+## for qmake projects 
+
+ * cd yourRepo
+ * git submodule add https://github.com/QuasarApp/QtBigInt.git # add the repository of Qt-Secret into your repo like submodule
+ * git submodule update --init --update
+ * Include in your pro file the pri file of Qt-Secret library
+  >> include($$PWD/QtBigInt/GMP.pri)
+ * Rebuild yuor project
+
+## for other build system
+ 
+ * cd yourRepo
+ * git submodule add https://github.com/QuasarApp/QtBigInt.git # add the repository of Qt-Secret into your repo like submodule
+ * git submodule update --init --update
+ * Add the rule for build Qt-Secret
+ * Add INCLUDEPATH and LIBS for your build system 
+ * Rebuild yuor project
+
+
+# Usage
+Create objects of the BigInt class, and do what you got to do!
+
+``` cpp
+#include <bigint.h>
+
+BigInt big1 = 1234567890, big2;
+big2 = "9876543210123456789098765432101234567890";
+
+std::cout << (big1 * big2 * 123456).getSring() << "\n";
+// Output: 1505331490682966620443288524512589666204282352096057600
+```