update to readme for apt-get install instructions

QCOR nightly docker images are available that serve up an Eclipse Theia IDE (the same IDE Gitpod uses) on port 3000. To get started, run 

```bash

$ docker run --security-opt seccomp=unconfined --init -it -p 3000:3000 qcor/qcor

docker run --security-opt seccomp=unconfined --init -it -p 3000:3000 qcor/qcor

```

Navigate to ``https://localhost:3000`` in your browser to open the IDE and get started with QCOR. 

## Dependencies

```

Compiler (C++14): GNU 6.1+, Clang 3.4+

CMake 3.9+ (for build)

XACC: see https://xacc.readthedocs.io/en/latest/install.html#building-xacc

LLVM/Clang [Syntax Handler Fork](https://github.com/hfinkel/llvm-project-csp).

```

- Compiler (C++17): GNU 8.4+, Clang 5+

- CMake 3.12+ (for build)

- XACC - [Build XACC](https://xacc.readthedocs.io/en/latest/install.html#building-xacc).

- LLVM/Clang - [Syntax Handler Fork](https://github.com/hfinkel/llvm-project-csp).

## Linux Build Instructions

Easiest way to install CMake - do not use the package manager,

instead use `pip`, and ensure that `/usr/local/bin` is in your PATH:

```bash

$ python3 -m pip install --upgrade cmake

$ export PATH=$PATH:/usr/local/bin

python3 -m pip install --upgrade cmake

export PATH=$PATH:/usr/local/bin

```

For now we require our users build a specific fork of LLVM/Clang that 

in a future release of LLVM and Clang, and at that point users will only 

need to download the appropriate LLVM/Clang binaries (via `apt-get` for instance).

To build this fork of LLVM/Clang (be aware this step takes up a good amount of RAM):

To build this fork of LLVM/Clang (be aware this step takes up a good amount of time / RAM):

```bash

$ apt-get install ninja-build [if you dont have ninja]

$ git clone https://github.com/hfinkel/llvm-project-csp llvm

$ cd llvm && mkdir build && cd build

$ cmake -G Ninja ../llvm -DCMAKE_INSTALL_PREFIX=$HOME/.llvm -DBUILD_SHARED_LIBS=TRUE -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD="X86" -DLLVM_ENABLE_DUMP=ON -DLLVM_ENABLE_PROJECTS=clang

$ cmake --build . --target install

$ sudo ln -s $HOME/.llvm/bin/llvm-config /usr/bin

apt-get install ninja-build [if you dont have ninja]

git clone https://github.com/hfinkel/llvm-project-csp llvm

cd llvm && mkdir build && cd build

cmake -G Ninja ../llvm -DCMAKE_INSTALL_PREFIX=$HOME/.llvm -DBUILD_SHARED_LIBS=TRUE -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD="X86" -DLLVM_ENABLE_DUMP=ON -DLLVM_ENABLE_PROJECTS=clang

cmake --build . --target install

sudo ln -s $HOME/.llvm/bin/llvm-config /usr/bin

```

Note that, for now, developers must clone QCOR manually:

To build QCOR from source:

``` bash

$ git clone https://github.com/ornl-qci/qcor

$ cd qcor

$ mkdir build && cd build

$ cmake .. 

$ [with tests] cmake .. -DQCOR_BUILD_TESTS=TRUE

$ make -j$(nproc) install

git clone https://github.com/ornl-qci/qcor

cd qcor

mkdir build && cd build

[Running cmake with no flags will search for LLVM using `llvm-config` executable in your PATH and XACC in $HOME/.xacc]

cmake .. 

   [Extra optional flags] 

   -DQCOR_BUILD_TESTS=TRUE

   -DLLVM_ROOT=/path/to/llvm

   -DXACC_DIR=/path/to/xacc

make -j$(nproc) install

```

Update your PATH to ensure that the ```qcor``` compiler is available.

```bash