Add instructions on how to run glmark2+ANGLE on Linux

doc/ExternalBenchmarks.md

+# Using External Benchmarks with ANGLE
+
+This document contains instructions on how to run external benchmarks on ANGLE as the GLES renderer.
+There is a section for each benchmark with subsections for each platform.  The general theme is to
+make the benchmark application pick ANGLE's `libGLESv2.so` and `libEGL.so` files instead of the
+system ones.
+
+On Linux, this is generally achieved with setting `LD_LIBRARY_PATH`.  On Windows, ANGLE dlls may
+need to be copied to the benchmark's executable directory.
+
+## glmark2
+
+This benchmark can be found on [github](https://github.com/glmark2/glmark2).  It's written against
+GLES 2.0 and supports Linux and Android.  It performs tens of tests and reports the framerate for
+each test.
+
+### glmark2 on Linux
+
+To build glmark2 on Linux:
+
+```
+$ git clone https://github.com/glmark2/glmark2.git
+$ cd glmark2
+$ ./waf configure --with-flavors=x11-glesv2 --data-path=$PWD/data/
+$ ./waf
+```
+
+To run glmark2 using the native implementation of GLES:
+
+```
+$ cd build/src
+$ ./glmark2-es2
+```
+
+To run glmark2 using ANGLE, we need to first create a few links in the build directory of ANGLE:
+
+```
+$ cd /path/to/angle/out/release
+$ ln -s libEGL.so libEGL.so.1
+$ ln -s libGLESv2.so libGLESv2.so.2
+```
+
+Back in glmark2, we need to make sure these shared objects are picked up:
+
+```
+$ cd /path/to/glmark2/build/src
+$ LD_LIBRARY_PATH=/path/to/angle/out/release/ ldd ./glmark2-es2
+```
+
+With `ldd`, you can verify that `libEGL.so.1` and `libGLESv2.so.2` are correctly picked up from
+ANGLE's build directory.
+
+To run glmark2 on the default back-end of ANGLE:
+
+```
+$ LD_LIBRARY_PATH=/path/to/angle/out/release/ ./glmark2-es2
+```
+
+To run glmark2 on a specific back-end of ANGLE:
+
+```
+$ ANGLE_DEFAULT_PLATFORM=vulkan LD_LIBRARY_PATH=/path/to/angle/out/release/ ./glmark2-es2
+```