Add contributing doc

CONTRIRUTING.md

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+# Writing code
+
+1. Download VS Code. it has typescript support out of the box, but any ide will
+   do. recommend downloading a typescript plugin if needed
+2. clone repo
+3. open repo in vs code
+4. run npm install
+5. start coding :)
+
+the code is in /src cmd.js is the entrypoint pipeline.js is the interesting high
+level functions (doRss, etc...)
+
+# Build and Run
+
+`npm run build` will compile the typescript files into javascript files in /dist
+`npm run watch` will compile, then incrementally compile when files change the
+`cross-seed` command is an alias for `node dist/cmd.js`, as in
+`node dist/cmd.js daemon` but it's annoying to use the cross-seed alias while
+developing so i use a lot of ctrl-R with fzf
+
+# Testing
+
+I keep a `config.js` that points to
+`{ torrentDir: "./input", outputDir: "./output" }` and put some torrent files in
+`./input` I usually run in search mode because it's the simplest form of
+cross-seed and most work applies to it.
+
+## testing daemon mode
+
+have three terminals open, one for npm run watch, one for keeping the cross-seed
+daemon open, and one for curling to localhost:2468
+
+## testing direct injection
+
+mostly i stick with `--save` (the default i think) usually i use docker to run
+whichever client. you could install it yourself though I have a gitignored
+docker_compose.yml that keeps track of all of the clients but i only run them
+one at a time