Update README.md with conda build instructions

# CSM-CameraModel

## Building with conda

## Building with conda (RECOMMENDED)

Install conda if you do not already have it.

```bash

source $HOME/.bashrc && \

which conda

```

> [bash installation reference](https://conda.io/docs/user-guide/install/linux.html "Reference to bash conda install")

> For more information: [bash installation](https://conda.io/docs/user-guide/install/linux.html "Reference to bash conda install")

### Setting up conda for tcsh

Copy and paste the following into a terminal running the `tcsh` shell:

source $HOME/.cshrc && \

which conda

```

> [tcsh installation reference](https://github.com/ESMValGroup/ESMValTool/issues/301 "Reference to tcsh conda install")

> For more information: [tcsh installation](https://github.com/ESMValGroup/ESMValTool/issues/301 "Reference to tcsh conda install")

### Creating an isolated conda environment

Run the following commands to create a self-contained dev environment for CSM-CameraModel (type `y` to confirm creation):

```bash

conda create -n csmdev -c usgs-astrogeology cmake libcsm

```

> For more information: [conda environments](https://conda.io/docs/user-guide/tasks/manage-environments.html)

#### Activating the environment

After creating the `csmdev` environment and installing cmake and libcsm into it, we need to activate it. Right now, cmake, libcsm, and their dependencies are isolated to a conda environment and we need to tell conda that we want to use it. The activation command depends on your shell.

* **tcsh**: `conda activate csmdev`

After you've set up conda, you can build CSM-CameraModel:

```bash

1. Fork `USGS-Astrogeology/CSM-CameraModel` if you don't already have a fork.

2. Clone your fork of `CSM-CameraModel` *with `--recursive` option to get the gtest submodule*.

```bash

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

cd CSM-CameraModel

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

```

3. Sync your fork with `upstream` and ensure the gtest submodule is init'd if your fork is old.

```bash

git pull upstream master

git submodule update --init --recursive

git push -u origin master

```

4. `mkdir build && cd build`

5. `cmake .. && make`

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