Update README.md

Author: eengl

README.md

@@ -43,3 +43,37 @@ Prior to ``make install ``, though optional, it is good practice to test the bui
 ### Install
 
 ``> [sudo] [FC=... FFLAGS="..." PREFIX="..."] make install`` **(sudo access required to install to system area)**
+
+## Usage
+
+This package provides a module file (**``<prefix>/include/strings.mod``**) and both a shared object library (**``<prefix>/lib/libfstrings.so``**) and a static library (**``<prefix>/lib/libfstrings.a``**).  To use this ``fortran-strings`` in your Fortran program, you must use the ``USE`` statement in your main program or procedure source and during compile, you must specify the library of your choice to the compiler/linker.
+
+Example code:
+```fortran
+program test
+use strings
+implicit none
+
+character(len=:), allocatable :: mystring
+integer :: icount
+
+mystring="Hello World!  Hello from Fortran!"
+icount=str_count(mystring,"Hello") ! Return a count of "Hello" in mystring
+write(6,*)"icount = ",icount
+
+end program test
+```
+
+Example compile and link to ***static*** library using gfortran:
+```bash
+gfortran -I<prefix>/include -o test.x test.f90 <prefix>/lib/libfstrings.a
+```
+
+Note that this does not make the executable 100% static.
+
+Example compile and link to ***shared object*** library using gfortran:
+```bash
+gfortran -I<prefix>/include -o test.x test.f90 -L<prefix>/lib -lfstrings
+```
+
+Note that when compiling and linking to shared object libraries, the library path must be specified in the appropriate environment variable prior to invocation (Linux: ``LD_LIBRARY_PATH``; macOS: ``DYLD_LIBRARY_PATH``).