Add install instructions (#135)

Author: cheesema

INSTALL_INSTRUCTIONS.md

@@ -0,0 +1,44 @@
+# Install instructions for LibAPR.
+
+The way to use it is configure it via some new APR_* variables (can be set to ON/OFF depending on needs) which describes what to build and if it should be installed through cmake commands:
+
+APR_BUILD_STATIC_LIB=ON
+APR_BUILD_SHARED_LIB=OFF
+APR_INSTALL=ON
+(all other configuration possibilities are now in the top of CMakeLists.txt file)
+
+so full command line would look like:
+
+```
+mkdir build
+cd build
+cmake -DCMAKE_INSTALL_PREFIX=/tmp/APR -DAPR_INSTALL=ON -DAPR_BUILD_STATIC_LIB=ON -DAPR_BUILD_SHARED_LIB=OFF ..
+make
+make install
+```
+
+To use APR the minimalistic CMakeLists.txt file would look like:
+
+```
+cmake_minimum_required(VERSION 3.2)
+project(myAprProject)
+set(CMAKE_CXX_STANDARD 14)
+
+#external libraries needed for APR
+find_package(HDF5 REQUIRED)
+find_package(TIFF REQUIRED)
+include_directories(${HDF5_INCLUDE_DIRS} ${TIFF_INCLUDE_DIR} )
+
+find_package(APR REQUIRED)
+
+add_executable(HelloAPR helloWorld.cpp)
+target_link_libraries(HelloAPR  ${HDF5_LIBRARIES} ${TIFF_LIBRARIES} apr::staticLib)
+```
+
+if shared version is preferred then apr::sharedLib should be used (and of course APR_BUILD_SHARED_LIB=ON during lib build step).
+
+NOTICE: if APR is isntalled in not standard directory then some hint for cmake must be provided by adding install dir to CMAKE_PREFIX_PATH like for above example:
+
+```
+export CMAKE_PREFIX_PATH=/tmp/APR:$CMAKE_PREFIX_PATH
+```