Updated readme with special isntructions for os Mojave

Author: Kyli McKay-Bishop

README.md

@@ -21,8 +21,20 @@ supported by all modern browsers.
 ```
     <../build>$ make -j8
 ```
+**BUILDING ON OS MOJAVE OR LATER**
 
+Apple made changes that removed the /usr/include file and now some programs won't compile, including dSpaceX. 
+The following steps provide instructions for adding the /usr/include file back.
+1. Check to see if you have Xcode command line tools installed
+    1. Start terminal
+    2. run command ``` xcode-select -p ```
+2. If you do not have Xcode command line tools installed follow these [directions](https://www.embarcadero.com/starthere/xe5/mobdevsetup/ios/en/installing_the_commandline_tools.html) 
+3. Run the following command in the terminal 
+``` 
+sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target /
+```
 
+**Additional Notes**
 The default CMake configuration will only build the HDProcess library.
 It can also optionally build the following binaries if desired:
 - HDViz           - A GUI for Visualizating Datasets.