Add hpx instructions

Author: NanmiaoWu

README.rst

@@ -159,6 +159,41 @@ installed using the following commands:
 
 where ``$GMT_ROOT`` is the directory where you want the library to be installed.
 
+HPX
+"""
+
+SHAD uses HPX as backend for commodity clusters. Software prerequisites for HPX 
+on Linux systems are: compiler, CMake, Boost C++ Libraries, and Portable Hardware
+Locality(HWLOC). You can find more details in `HPX Build Recipes <https://stellar-group.github.io/hpx/docs/sphinx/latest/html/manual/building_hpx.html#building-hpx>`_. 
+
+It can be installed using the following commands:
+
+.. code-block:: shell
+
+    $ git clone https://github.com/STEllAR-GROUP/hpx.git
+    $ cd hpx
+    $ git checkout 9a83230605142392c34c81c0902eb5e9ae9bf973
+    $ mkdir build && cd build
+    $ cmake .. -DCMAKE_INSTALL_PREFIX=$HPXOOT                          \
+        -DHPX_WITH_CXX17=ON                                            \
+        -DCMAKE_CXX_STANDARD=17                                        \
+        -DHPX_USE_CMAKE_CXX_STANDARD=ON                                \
+        -DHPX_SERIALIZATION_WITH_ALL_TYPES_ARE_BITWISE_SERIALIZABLE=On \
+        -DHPX_SERIALIZATION_WITH_ALLOW_RAW_POINTER_SERIALIZATION=On    \
+        -DHPX_SERIALIZATION_WITH_ALLOW_CONST_TUPLE_MEMBERS=On          \
+        -DHPX_WITH_PARCELPORT_MPI=ON                                   \
+        -DHPX_WITH_PARCELPORT_TCP=OFF                                  \
+        -DHPX_WITH_HWLOC=ON                                            \
+        -DHPX_WITH_MALLOC=tcmalloc                                     \
+        -DCMAKE_BUILD_TYPE=Release                                     \
+        -DHWLOC_ROOT=$HWLOCROOT                                        \
+        -DTCMALLOC_ROOT=$GPERFTOOLSROOT                                \
+        -DBOOST_ROOT=$BOOSTROOT                                        \
+        
+    $ make -j <SOMETHING_REASONABLE> && make install
+
+where ``$HPXROOT`` is the directory where you want the library to be installed.
+
 Build SHAD
 ----------
 
@@ -182,6 +217,8 @@ provide additional backends. Target runtime systems may be specified via the
         -DTBB_ROOT=$TBBROOT                            \
         # else if using GMT                            \
         -DGMT_ROOT=$GMTROOT                            \
+         # else if using HPX                           \
+        -DHPX_DIR=$HPXROOT                            \
         # endif                                        \
         -DGTEST_ROOT=$GTESTROOT                        \
         -DGPERFTOOLS_ROOT=$GPERFTOOLSROOT