docs: add documentation about repository file

docs/repository-list-file-format.md

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+# Repository List file format
+
+This document describes the format of the repository list file used in ALCOM / vrc-get.
+
+## File format
+
+1. The file is a UTF-8 encoded text file.
+2. For each line, text after '#' will be ignored as a comment.
+3. Each line contains a repository URL, or empty.
+4. Each line is trimmed before processing. (this mean any line only with spaces will be ignored)
+5. Each repository line should only contain a valid URL.
+6. The URL schema must be `http`, `https`, or `vcc`.\ 
+   Other schemas are ignored and might be recognized for other purposes in the future.
+7. If the URL is a `http` or `https` URL, the URL represents a VPM repository without headers.
+8. If the URL is a `vcc` URL, the URL should be a VCC URL to add VPM repository, which is described below.\
+   This notation is used to express the repository with headers.
+
+## VCC URL format
+
+The VCC URL to add VPM Repository will be a valid URL with the following format:
+
+- schema must be `vcc`
+- the host part must be `vpm`
+- the path part must be `/addRepo`
+- the query part must contain single `url` parameter which represents the repository URL to add.
+- the query part may contain `headers[]` parameter with represents the HTTP headers for the repository.
+  - query value will be split by `:` and prior part will be the header name and the rest will be the header value.
+
+## Examples
+
+```text
+# This is a comment
+http://example.com/repo
+https://example.com/repo
+
+vcc://vpm/addRepo?url=http://example.com/repo&headers[]=header-name:header-value
+```
+
+This file represents a repository list with the following repositories:
+
+- `http://example.com/repo`
+- `https://example.com/repo`
+- `http://example.com/repo` with a custom header `header-name:header-value`
+
+Another example may be found at the `repositories.txt` at the root of this repository.