Update 'README.md'

README.md

@@ -1,6 +1,14 @@
 # Changed version of ORB_SLAM3 for Windows 10.
 Project is provided with already built ORB_SLAM3.lib library. You might want to skip the build part and put the library from `lib/ORB_SLAM3.lib` to `build/Release/ORB_SLAM3.lib`
 
+## If you downloaded ORB_SLAM3 from the official repository, all you need to do is: 
+1. Add unistd.h and getopt.c and getopt.h to include/ folder. 
+2. Add `void usleep(__int64 usec);` to `Sytem.h` before `namespace ORB_SLAM3`, and add: 
+void usleep(__int64 usec){
+    std::this_thread::sleep_for(std::chrono::microseconds(usec));
+}
+to the `System.cc`file before `namespace ORB_SLAM3`. 
+
 ## Prerequisites
 - OpenCV3 (Not from Anaconda): https://opencv.org/releases/
 - Boost (You need to build it): https://www.boost.org/users/download/