Update build instructions

[CiaranWelsh]

Hello,

I've noted that some recent changes in libcombine make the build instructions on the github page obsolete. Notably, libsbml and its dependencies are now included in libCombine as submodules alongside zipper. In order to build libCombine I had to first build the submodules - which was pretty easy. Here are the commands I used:

# get the repo - remembering to recursively clone the submodules
$ git clone --recurse-submodules https://github.com/sbmlteam/libCombine.git

# build libsbml-dependencies package first
$ cd libCombine/submodules/libSBML-dependencies
$ mkdir build && cd build
$ cmake -DCMAKE_INSTALL_PREFIX=../../libSBML/dependencies ..
$ make -j 8
$ make install

# now build libsbml
$ cd ../../libSBML
$ mkdir build && cd build 
$ cmake ..
$ make -j 8
$ sudo make install

# now build zipper
$ cd ../../zipper 
$ mkdir build && cd build 
$ cmake .. 
$ make -j 8 
$ sudo make install 

# and now build libCombine
$ cd ../../.. 
$ mkdir build && cd build 
$ cmake ..
$ make -j 8
$ sudo make install 

Ciaran