Support/instructions for Pipx install

I use [pipx](https://pipxproject.github.io/pipx/) to manage Python executables on my system and would like to use it to install Sclack. I tried running the following command in Pipx:
`pipx install --include-deps --spec git+https://github.com/haskellcamargo/sclack.git sclack` which seemed to have worked, however, it installed Sclack as a binary in my path `~/.local/bin` named `app.py` rather than `sclack` and when I attempt to run `app.py` (which is exposed on my path) I get the following errors:

```
Traceback (most recent call last):
File "/home/michael/.local/bin/app.py", line 14, in
from sclack.components import Attachment, Channel, ChannelHeader, ChatBox, Dm
File "/home/michael/.local/pipx/venvs/sclack/lib/python3.7/site-packages/sclack/components.py", line 11, in
from .markdown import MarkdownText
File "/home/michael/.local/pipx/venvs/sclack/lib/python3.7/site-packages/sclack/markdown.py", line 3, in
from .store import Store
File "/home/michael/.local/pipx/venvs/sclack/lib/python3.7/site-packages/sclack/store.py", line 1, in
from slackclient import SlackClient
ModuleNotFoundError: No module named 'slackclient'
```