Update README.md with more install instructions

To build:

1. Ensure that the CSM is installed in a place that CMake searches (e.g., /usr or /usr/local).

2. `mkdir build` && `cd build`

3. `cmake .. && make`

 No newline at end of file

1. Install [libcsmapi](https://github.com/sminster/csm "CSM API")

    > You can install this with an `INSTDIR` of your choice, or let it default (see [libcsmapi README](https://github.com/sminster/csm/blob/master/README))

```bash

mkdir $HOME/csmenv

cd $HOME

git clone git@github.com:sminster/csm.git

cd csm

make -f Makefile.linux64 all install clean INSTDIR="$csmenv"

```

2. Install cmake >= 3.10

```bash

cd $HOME

wget https://cmake.org/files/v3.10/cmake-3.10.0-Linux-x86_64.tar.gz

tar xzf cmake-3.10.0-Linux-x86_64.tar.gz

rsync -azv cmake-3.10.0-Linux-x86_64/ $HOME/csmenv/

echo -e "\n#Prepending csm env to path\nsetenv PATH "$HOME/csmenv/bin:$PATH" >> $HOME/.cshrc

source $HOME/.cshrc

```

3. Fork and clone down this repo and its submodules (gtest)

```bash

git clone --recursive git@github.com:<username>/CSM-CameraModel.git

cd CSM-CameraModel

git remote add upstream git@github.com:USGS-Astrogeology/CSM-CameraModel.git

git pull upstream master

git submodule update --init --recursive

git push -u origin master

```

4. `mkdir build` && `cd build`

5. `cmake -DCSM_INCLUDE_DIR="${csmenv}/include/csm -DCSM_LIBRARY="${csmenv}/lib/libcsmapi.so .. && make`