message: Update installation docs for protobuf support
  Summary: Updated the docs to let users know how to enable protobuf to support AppBundle when use autoconf to install Redex.
diff: @@ -19,10 +19,11 @@ Install dependencies using homebrew:
  brew install autoconf automake libtool python3
  brew install boost jsoncpp
  ```
  +For App Bundle support `brew install protobuf` is also required.
  ### Ubuntu/Debian (64-bit)
  Base requirements are automake & libtool, GCC >= 7, Python >= 3.6 and Boost >= 1.71.0, as well as
  -development versions of `iberty`, `jemalloc`, `jsoncpp`, `lz4`, `lzma`, and `zlib`.
  +development versions of `iberty`, `jemalloc`, `jsoncpp`, `lz4`, `lzma`, and `zlib`. `Protobuf` >= 3.0 is required if optimizing an App Bundle.
  #### Ubuntu 18.04+, Debian 10(Buster)+
  The minimum supported Ubuntu version is 18.04. The minimum supported Debian version is 10.
  @@ -32,6 +33,7 @@ on older OS versions.
  ```
  sudo ./setup_oss_toolchain.sh
  ```
  +After the script, please run `sudo ldconfig` if it throws an error about loading shared libraries for running protoc.
  ### Experimental: Windows (64-bit) with MSYS2
  @@ -79,6 +81,22 @@ Now, build ReDex using autoconf and make.
  autoreconf -ivf && ./configure && make
  sudo make install
  ```
  +
  +Alternatively, to enable protobuf to support App Bundles, please use:
  +```
  +autoreconf -ivf && ./configure --enable-protobuf
  +make
  +sudo make install
  +```
  +
  +For protobuf installed in the default system (Homebrew) search path (e.g /usr/local/bin), `--enable-protobuf` is sufficient to trigger the build. Otherwise, specify the protobuf installation path for the autoconf:
  +```
  +autoreconf -ivf
  +./configure --with-protoc=/path/to/protoc --with-protolib=/path/to/protobuf_libs --with-protoheader=/path/to/protobuf_headers --enable-protobuf
  +make
  +sudo make install
  +```
  +
  *If your build machine has lots of RAM (on the order of 2-4GB per core), using
  Make parallelism can speed up the build (e.g., `make -j4`). However, the C++
  compilers are very memory hungry and this needs to be finely tuned on many
  