Fixed underlining in README.md and added missing Debian instructions

[felix91gr]

• Underlining of the heading links in README covered one space after the heading. That is now fixed, and the underlinings end at the exact end of their respective headings.
• Some Debian requirements were missing (namely, tesserocr's package dependencies and a particular dependency of spyder that is needed in Debian systems due to a known issue: https://docs.spyder-ide.org/current/installation.html#using-pip)

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I hope I formatted the PR's commits properly. Do tell me if the commits need splitting or if the commit messages should be changed.

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Btw, this project will be extremely useful to me. I'm working on a manga translation, and I've just tested Poricom on about 55 of its pages with text and had no misses. (Insert "technology is incredible" meme here, holy shit)

[blueaxis]

I think pyqt5-dev-tools is only required if you're using spyder IDE so please mention that in the README.

As for formatting, since the requirements are more of a dev setup, can you please change it like so:

### Development Setup
...

#### Linux (Debian / Ubuntu)
[CHANGES]

[felix91gr]

I think pyqt5-dev-tools is only required if you're using spyder IDE so please mention that in the README.

I don't fully understand why the following was the case, but the project wouldn't run without having spyder available as a library, which itself required the pyqt5-dev-tools.

This is true, even though I'm not even using the IDE myself. I don't even have it installed.

It might be a quirk with the dependencies required to run the project on Linux. I'm not sure. I'm on the latest Ubuntu.

[felix91gr]

As for formatting, since the requirements are more of a dev setup, can you please change it like so: (...)

Ah. That makes sense. Sorry, I got into troubleshooting the dev setup because the Linux version isn't yet packaged in the released zip files.

I forgot this is the dev setup and not the "normal user" setup. Thanks for pointing it out :)

I'll work on reformatting it tomorrow.