Add documentation for the annotations command

Signed-off-by: Alexandre Terrasa <alexandre.terrasa@shopify.com>

README.md

@@ -291,6 +291,30 @@ Nothing to do, all RBIs are up-to-date.
 
 This option can be used on CI to make sure the RBI files are always up-to-date and ensure accurate type checking. **Warning**: doing so will break your normal Dependabot workflow as every pull-request opened to bump a gem version will fail CI since the RBI will be out-of-date and will require you to manually run `bin/tapioca gems` to update them.
 
+### Pulling RBI annotations from remote sources
+
+Since Tapioca does not perform any type inference, the RBI files generated for the gems do not contain any type signatures. Instead, Tapioca relies on the community to provide high-quality, manually written RBI annotations for public gems.
+
+To pull the annotations relevant to your project from the central repository, run the `annotations` command:
+
+```shell
+bin/tapioca annotations
+```
+
+By default, Tapioca will pull the annotations stored in the central repository located at https://github.com/Shopify/rbi-central. It is possible to use a custom repository by changing the value of the `--sources` options. For example if your repository is stored on Github:
+
+```shell
+bin/tapioca annotations --sources https://raw.githubusercontent.com/$USER/$REPO/$BRANCH
+```
+
+Tapioca also supports pulling annotations from multiple sources:
+
+```shell
+bin/tapioca annotations --sources https://raw.githubusercontent.com/$USER/$REPO1/$BRANCH https://raw.githubusercontent.com/$USER/$REPO2/$BRANCH
+```
+
+> Note: support for private repositories is not yet implemented.
+
 ### Generating RBI files for Rails and other DSLs
 
 Sorbet by itself does not understand DSLs involving meta-programming, such as Rails. This means that Sorbet won't know about constants and methods generated by `ActiveRecord` or `ActiveSupport`.