Merge pull request udacity#2 from roman-smirnov/patch-1

README: add mac caveat to install instruction

README.md

@@ -20,6 +20,7 @@ See the classroom instruction and code comments for more details on each of thes
   * Windows: [Click here for installation instructions](http://gnuwin32.sourceforge.net/packages/make.htm)
 * OpenCV >= 4.1
   * This must be compiled from source using the `-D OPENCV_ENABLE_NONFREE=ON` cmake flag for testing the SIFT and SURF detectors.
+  * If using [homebrew](https://brew.sh/): `$> brew install --build-from-source opencv` will install required dependencies and compile opencv with the `opencv_contrib` module by default (no need to set `-DOPENCV_ENABLE_NONFREE=ON` manually). 
   * The OpenCV 4.1.0 source code can be found [here](https://github.com/opencv/opencv/tree/4.1.0)
 * gcc/g++ >= 5.4
   * Linux: gcc / g++ is installed by default on most Linux distros
@@ -31,4 +32,4 @@ See the classroom instruction and code comments for more details on each of thes
 1. Clone this repo.
 2. Make a build directory in the top level directory: `mkdir build && cd build`
 3. Compile: `cmake .. && make`
-4. Run it: `./2D_feature_tracking`.
+4. Run it: `./2D_feature_tracking`.