build instructions and other comments

README.md

@@ -5,3 +5,32 @@ Apache DataSketches is an open source, high-performance library of stochastic st
 This code requires C++11. It was tested with GCC 4.8.5 (standard in RedHat at the time of this writing), GCC 8.2.0 and Apple LLVM version 10.0.1 (clang-1001.0.46.4)
 
 This includes Python bindings. For the Python interface, see the README notes in [the python subdirectory](https://github.com/apache/incubator-datasketches-cpp/tree/master/python).
+
+This library was intended to be header-only, but this goal was not fully
+achieved yet with CPC sketch code. This work is in progress.
+
+Building and running unit tests requires CppUnit.
+
+Installing CppUnit on OSX: brew install cppunit
+
+Installing CppUnit on RHEL: yum install cppunit-devel
+
+There are currently two ways of building: using existing make files and generating
+make files using cmake. Exsisting make files might not work on all platforms
+or with all C++ compilers. Generating make files using cmake should solve
+this problem, but it currently requires cmake version 3.12.0 or later that might not
+be readily available as a package on all platforms.
+Installing latest cmake on OSX: brew install cmake
+
+Building and running unit tests using existing make files:
+
+	$ make
+	$ make test
+
+Building and running unit tests using cmake:
+
+	$ mkdir build
+	$ cd build
+	$ cmake ..
+	$ make
+	$ make test