add READING file

READING.md

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+This file provides an overview of what functionality goes where.
+
+As convention for this file, when a path is given that does **not** start with a slash, its a relative path.
+Look further up in the section to find the "base path" it is relative to.
+
+## backend
+
+All the backend code is in `/packages/backend/src`.
+
+### Database
+
+For connecting to the database an ORM (object–relational mapping) is used.
+The ORM can map between database entries and TypeScript objects.
+The type definitions for these objects can be found in `/packages/backend/src/models/`.
+
+Definitions for individual objects are defined in `entities/`.
+Some behaviour that is mostly used by the API is defined in `repositories/`.
+These can be thought of like the database tables themselves.
+Often there is functionality called "pack" in these, and by "packing" a database object you will get the API representation of that object.
+Schemas for these API versions of the objects are defined in `schema/`.
+
+### API
+
+API stuff goes in `/packages/backend/src/server/api/`
+
+All API routes are "mounted" in `index.ts`.
+For this to work, the endpoint has to be tied into the respective path in `endpoints.ts`.
+
+Endpoints are defined using a JSON object, and documentation is automatically generated using these objects.
+The type definition for that object is also contained/documented in `endpoints.ts`.
+
+Reading that file you can see that the endpoint behaviour is defined in `endpoints/`.
+The paths below that directory should be the same as on the API.
+
+Handling the API calls is done in `api-handler.ts`, which does some preprocessing of the request.
+It then uses `authenticate.ts` to try to authenticate the user.
+After that the actual endpoint code is run by `call.ts` after checking some more of the preconditions set by the endpoint definiton.
+
+### ActivityPub
+
+ActivityPub related code is in `/packages/backend/src/remote/activitypub/`
+
+Incoming ActivityPub requests are processed by the code in `kernel/`.
+The entry point for processing an activity is `processOneActivity` in the `kernel/index.ts` file in that directory.
+Parts of incoming activities may also be processed by `models/`.
+
+The bodys of outgoing ActivityPub requests are "rendered" using `renderer/`.
+These files define several functions that are meant to be used together, e.g. `renderCreate(await renderNote(note, false), note)`.
+The invocation of these functions is placed either in the API endpoints directly or in the services code.
+
+Both incoming and outgoing ActivityPub request are handled through queues, to e.g. allow for retrying a request when it fails, or spikes of many incoming requests.
+
+### Services
+
+If code for handling a task seems to big to e.g. do directly in an API endpoint or should be reused, it is probably placed in `/packages/backend/src/services/`.
+
+### Queues
+
+Code regarding queues is in `/packages/backend/src/queue/`.
+
+Misskey defines several queues for processing things that might take longer in the background.
+The queues themselves are defined in `index.ts`.
+How the different queues are then processed is defined by the code in the `processors/` directory.
+
+## client
+
+All the client code is in `/packages/client/src/`.
+The client uses Vue 3 and its Composition API, but there are also some older Options API components that have yet to be refactored (see CONTRIBUTING.md).
+
+### Pages
+
+If it is something you can navigate to, it is most likely a page.
+These are defined in `/packages/client/src/pages/` and should be organized by the path you use to see them.
+
+### Components
+
+If it is not a page, it is probably a component.
+Components can be things like the emoji picker, a tooltip, a widget.
+Components can be combined to make bigger components or also pages.
+They are defined in `/packages/client/src/components/`.
+
+### Directives
+
+Vue has "directives", e.g. you can add an attribute to a component like `v-tooltip`.
+There are some built in directives like e.g. `v-if` and `v-for`.
+Other custom directives are defined in `/packages/client/src/directives/`.
+They are added to the Vue app in `index.ts` and their behavior is defined in separate files in the directory.