add docs for local development env

doc/source/local_environment.md

+# Local Development Environment
+
+The local development workflow consists of a few key points:
+
+- Managed conda environments using [conda-flow](https://git.ligo.org/james.kennington/conda-flow)
+- Using integrated development environments (IDEs) like [PyCharm]()/[CLion]()
+- Running applications that can consume `gstlal`, like [Jupyter Notebook]()
+
+## Creating the environment
+
+Thanks to conda-flow, creating the environment is simple. Before we can use conda-flow, it must be installed. In
+whatever environment you prefer, install conda-flow from pip:
+
+```bash
+pip install conda-flow
+```
+
+Once conda-flow is installed, There are locked environment files contained within the gstlal repo under
+`gstlal/gstlal/share/conda/envs`. Using conda-flow, we can make sure the local development conda environment is built:
+
+```bash
+conda-flow activate -n gstlal-dev -c /path/to/gstlal/gstlal/share/conda/conda-flow.yml
+```
+
+## Activating the environment
+
+The `activate` command within conda-flow is done through subprocesses, and consequently will *not* affect the parent
+process, such as the shell from which conda-flow is called. This is done to prevent unintended side effects; however, it
+also means that unlike `conda activate`, `conda-flow activate` will not activate the environment inside the shell. If
+you wish to activate the environment inside the shell, run `conda activate gstlal-dev`.
+
+## Using Developer Tools
+
+To use an IDE to develop `gstlal`, you will first need to start your IDE from within the appropriate conda environment.
+For example, to launch the PyCharm IDE, run:
+
+```bash
+conda-flow activate -n gstlal-dev --run-after "open -na ""PyCharm.app""" -c /path/to/gstlal/gstlal/share/conda/conda-flow.yml
+```
+
+### Python Development
+
+Note that the python source modules are not in a typical pythonic package structure (due to use of GNU build for c code
+within gstlal). This can present problems with package indexing / imports in the IDE. The solution is to provide the IDE
+with a map of the proper import paths, which can be done via two ways:
+
+1. Build `gstlal` (or at least the python components) and add the build directory as a source directory in your IDE
+1. Create a new source directory full of symlinks to the source files with a structure that mimicks the import paths.
+   There is a utility for constructing such symlinks at `gstlal/gstlal/share/conda/dev_links.py`
+
+## Launching Applications
+
+Conda-flow is capable of running arbitrary commands after activating the environment in the subprocess which is useful
+for launching applications in a controlled way. For example, to run a jupyter notebook:
+
+```bash
+conda-flow activate -n gstlal-dev --run-after "jupyter notebook" -c /path/to/gstlal/gstlal/share/conda/conda-flow.yml
+```
+