doc/dev/macos: add doc for building on MacOS

Signed-off-by: Kefu Chai <kchai@redhat.com>

doc/dev/macos.rst

@@ -0,0 +1,42 @@
+build on MacOS
+==============
+
+Since we've switched to C++ 17, and the default clang shipped with Xcode 9 does not support all the C++ 17 language features, it's suggested to install clang using brew::
+
+  brew install --with-toolchain llvm
+
+and install all the necessary bits::
+
+  brew install nss snappy ccache cmake pkg-config
+  pip install cython
+
+install FUSE if you want to build the FUSE support::
+
+  brew cask install osxfuse
+
+apply the patch at https://gist.github.com/tchaikov/c3f324a7c36fc9774739cea319d5c49b , to address https://public.kitware.com/Bug/view.php?id=15943 . We cannot bump up the required cmake version yet, because RHEL/CentOS does not have the newer cmake yet.
+
+then, under the source directory of Ceph::
+
+  mkdir build
+  cd build
+  PKG_CONFIG_PATH=/usr/local/Cellar/nss/3.33/lib/pkgconfig \
+    CC=/usr/local/opt/llvm/bin/clang \
+    CXX=/usr/local/opt/llvm/bin/clang++ \
+    cmake .. -DBOOST_J=4 \
+    -DENABLE_GIT_VERSION=OFF \
+    -DWITH_EMBEDDED=OFF \
+    -DWITH_MANPAGE=OFF \
+    -DWITH_LIBCEPHFS=OFF \
+    -DWITH_XFS=OFF \
+    -DWITH_KRBD=OFF \
+    -DWITH_LTTNG=OFF \
+    -DWITH_BABELTRACE=OFF \
+    -DWITH_BLUESTORE=OFF \
+    -DWITH_RADOSGW=OFF \
+    -DWITH_SPDK=OFF \
+    -DSNAPPY_ROOT_DIR=/usr/local/Cellar/snappy/1.1.7
+
+The paths to ``nss`` and ``snappy`` might vary if newer versions of the packages are installed.
+
+Currently, the most practical uses for Ceph on MacOS might be FUSE and some other librados based applications.