vim-proj-spell is a vim plugin which enables project-specific spell files

Install vim-proj-spell using any one of the many plugin managers out there. Personally, I use vim-plug, so my ~/.vimrc has this line:

Plug 'alkim0/vim-proj-spell'

Create a .spell directory in your project root directory, then add a wordlist file. The name of the wordlist file does not matter, all wordlists in .spell are sourced. The wordlist is expected to be a single word per line.

Then open any file in the project, and the words should already be added to your list of spellfiles. If you manually changed the wordlist files in .spell, you can call :ProjSpellReload to resource the wordlist files.

Because, vim-proj-spell messes with the spellfile setting, if you do not have spellfile specified, when you use zg, the word may not be saved to a permanent location. To deal with this, either explicitly set your spellfile, or remap:

nnoremap <silent> zg :ProjSpellAddWordGlobal expand('<cword>')<cr>

I like zG to save words to the project-specific wordlist, so I also have:

nnoremap <silent> zG :ProjSpellAddWordProj expand('<cword>')<cr>

Note that this plugin will likely only work for Unix-like systems.