Update README.md with the saga of the build

README.md

@@ -25,3 +25,56 @@ Useful flags:
 * `--output <OUTPUT_FILE>` - a png file will be produced at the given location with the contents of the rendered shader (default is `output.png`)
 * `--vertex <PATH_TO_VERTEX_SHADER>` - provide a custom vertex shader file rather than using the default (provided in `get_image.cpp`).
 
+
+## Building
+
+Building the project uses CMake.
+
+First you must ensure you have the depdencies. There are scripts in buildscripts
+intended to run on the CI that can also be used to build locally. You can run
+these as follows: 
+
+On Windows:
+
+```bash
+bash buildscripts\1-install-deps-appveyor.sh
+```
+
+On Linux or OSX:
+
+```bash
+bash buildscripts/1-install-deps-travis.sh
+```
+
+(This will probably fail with an error at the last step, but all that matters is that the deps are created)
+
+Once you have the deps downloaded, you can build with CMake as follows:
+
+```bash
+mkdir build
+cmake ..
+cmake --build .
+```
+
+Note that if you are building on Windows you must have Visual Studio installed and you must specify the
+build generator explicitly:
+
+```bash
+cmake -G "Visual Studio 15 2017 Win64"  -T v140 ..
+```
+
+The reason for this is that if you do not specify Win64 it will default to 32-bit (which is incompatible
+with the libraries in deps). The "-T v140" flag specifies that it should build with a VS2015 C++ compiler
+(which is the same one the libraries are built with).
+
+If this complains about being unable to install a compiler and you do have Visual Studio installed, then
+you will need to install Visual C++ support for VS2015. You can obtain the build tools from
+https://www.visualstudio.com/ by signing up for the Microsoft Developer Essentials (it's free).
+Note that you don't have to install the entirety of VS2015 (which is much larger).
+
+It *may* work correctly if you simply omit the "-T v140" flag, but we think that might be a bad idea
+but are not entirely sure.
+
+Note that you will see an awful lot of warnings. This is fine. As long as the build exits successfully,
+try running the generated executable (in either Debug or Release in your build directory) on sample.frag
+in the root of this project. If you get a pretty image called output.png then it has worked.