Fixed gitignore to not ignore the build instructions documentation file.

.gitignore

@@ -1,4 +1,4 @@
-build*
+/build*
 *.kdev4
 *.csv
 *.ods

doc/build.md

@@ -0,0 +1,25 @@
+# Build instructions {#build}
+
+The library can be built using the [CMake build system](https://cmake.org/).
+A simple command sequence on Linux looks like this:
+
+ mkdir build
+ cd build
+ cmake ..
+ make
+
+Below you can find a list of all parameters for building CMR.
+As an optional step, one can execute unittests:
+
+ make test
+
+The executables now reside in the current directory. One can also install them via
+
+ make install
+
+**Build Parameters**
+
+ - `-DCMAKE_BUILD_TYPE=Release` Compiles the code with optimization turned on. Make sue to use this if you need fast code for large matrices.
+ - `-DGENERATORS=on` Builds [generator tools](\ref generators) for certain matrices.
+ - `-DGMP=off` Disables large numbers; see \ref equimodular.
+