Update readme with build instructions

README.txt

@@ -10,3 +10,14 @@ The libovni.so library is licensed under MIT, while the rest of tools are GPLv3
 unless otherwise stated.
 
 For more information, take a look at the doc/ directory.
+
+To build ovni you would need a C compiler, MPI and cmake version 3.10 or newer.
+To compile in build/ and install into $prefix use:
+
+  % cmake -DCMAKE_INSTALL_PREFIX:PATH=$prefix -S . -B build
+  % cd build
+  % make
+  % make install
+
+See cmake(1) and cmake-env-variables(7) to see more information about
+the variables affecting the generation and build process.