Add development guide

[christian-byrne]

Adds guide for contributors.

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NOTE: This guide applies to developing after Manager v4 is fully released. Before that time, you need to follow these extra steps to test Manager v4 beta:

# checkout backend PR branch
cd ComfyUI
git remote add ltdrdata https://github.com/ltdrdata/ComfyUI.git
git fetch ltdrdata
git checkout ltdrdata/dr-support-pip-cm
# checkout frontend PR branch
git clone https://github.com/Comfy-Org/ComfyUI_frontend.git
cd ComfyUI_frontend
git checkout manager/menu-items-migration
# build frontend PR branch
npm install
export ALGOLIA_APP_ID=4E0RO38HS8
export ALGOLIA_API_KEY=684d998c36b67a9a9fce8fc2d8860579
npm run build
cd ..
# install manager pip package
pip install comfyui_manager==4.0.0b1
# start ComfyUI
python main.py --auto-launch --front-end-root ComfyUI_frontend/dist
# find the 'Manager' item in the topbar menu