Update install.md and add installation instructions for gtest

INSTALL.md

@@ -1,4 +1,7 @@
-libcarbon uses [CMake](https://cmake.org) as build system. CMake 3.9.6 or higher is required. For testing, [Google Test Framework](https://github.com/google/googletest) is required.  
+libcarbon uses [CMake](https://cmake.org) as build system. CMake 3.9.6 or higher is required. 
+For testing, [Google Test Framework](https://github.com/google/googletest) is required. You will find
+installation instructions in [misc/INSTALL_GTEST.md](misc/INSTALL_GTEST.md). Libcarbon is built for the [Clang C Compiler](https://clang.llvm.org). 
+To install this compiler on Linux, use `$ sudo apt install clang` if not yet present.  
 
 The basic usage is 
 ```

misc/INSTALL_GTEST.md

@@ -0,0 +1,21 @@
+Taken from [https://www.eriksmistad.no/getting-started-with-google-test-on-ubuntu/](https://www.eriksmistad.no/getting-started-with-google-test-on-ubuntu/)
+
+```
+sudo apt-get install libgtest-dev
+
+sudo apt-get install cmake # install cmake
+
+cd /usr/src/gtest
+
+sudo cmake CMakeLists.txt
+
+sudo make
+
+sudo cp *.a /usr/lib
+
+sudo mkdir /usr/local/lib/gtest
+
+sudo ln -s /usr/lib/libgtest.a /usr/local/lib/gtest/libgtest.a
+
+sudo ln -s /usr/lib/libgtest_main.a /usr/local/lib/gtest/libgtest_main.a
+```