Add basic build instructions

readme.md

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+# How to build
+
+To customize your build, you may change several variables that are consumed by make.
+
+These variables, along with some explanations can be found in Makefile.conf.
+
+You can edit that file, or create a file `local.conf` in the root of the repository, which will also be parsed by make.
+
+A sample `local.conf` looks like this:
+
+~~~
+PREFIX=$(HOME)/code/libad/build
+
+ENGINE_LIB=-L$(HOME)/code/engine/build/lib
+ENGINE_INC=-I$(HOME)/code/engine/build/include
+~~~
+
+Now, run:
+
+~~~
+make all
+make install
+~~~