document how to accelerate the download with aria2

README.md

@@ -351,6 +351,9 @@ name | default | description
 `PYENV_DEBUG` | | Outputs debug information.<br>Also as: `pyenv --debug <subcommand>`
 `PYENV_HOOK_PATH` | [_see wiki_][hooks] | Colon-separated list of paths searched for pyenv hooks.
 `PYENV_DIR` | `$PWD` | Directory to start searching for `.python-version` files.
+`PYTHON_BUILD_ARIA2_OPTS` | | Used to pass aditional parameters to [`aria2`](https://aria2.github.io/).<br>if `aria2c` binary is available on PATH, pyenv use `aria2c` instead of `curl` or `wget` to download the Python Source code. If you have an unstable internet connection, you can use this variable to instruct `aria2` to accelerate the download.<br>In most cases, you will only need to use `-x 10 -k 1M` as value to `PYTHON_BUILD_ARIA2_OPTS` environment variable
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 ## Development