add instructions to build to main readme

README.md

@@ -1,10 +1,23 @@
 # pressio-demoapps
 
-Suite of demo/mini apps. 
-While the primary purpose of this repository is to contain mini-apps 
-to use for testing ROMs in Pressio, this repository is self-contained in the sense 
+Suite of demo/mini apps.
+While the primary purpose of this repository is to contain mini-apps
+to use for testing ROMs in Pressio, this repository is self-contained in the sense
 that the code here does *not* depend on Pressio.
 
+# Building Tests
+Requires CMake > 3.18.0 and a C++ compiler with C++11 support:
+
+```
+git clone [email protected]:Pressio/pressio-demoapps.git
+
+export CXX=<path-to-your-CXX-compiler>
+cd pressio-demoapps && mkdir build && cd build
+cmake -DPRESSIODEMOAPPS_ENABLE_TESTS=On ..
+make -j4
+ctest -j4
+```
+
 # License and Citation
 [![License](https://img.shields.io/badge/License-BSD%203--Clause-blue.svg)](https://opensource.org/licenses/BSD-3-Clause)