Please make it explicit that 32bits are not supported

How I wasted 2h of my life:

I started tutorial by going to the page: https://github.com/Gallopsled/pwntools-tutorial/blob/master/installing.md
I had a VM with 32-bit Ubuntu 16 LTS, I used it to solve challenges from OverTheWire (they are mostly 32 bit).

I started with:
```
pip install --upgrade git+https://github.com/Gallopsled/pwntools.git
```
and it failed, looks like I need some other libraries, ok no problem:
```
sudo apt install libffi-dev
sudo apt install libssl-dev
```
But then pip cannot install cryptography module, it was failing with strange compile time errors.

OK probably old OpenSSL version (and BTW installing Python packages is getting more terrible than compiling C code). I compiled a new version from the sources: https://cloudwafer.com/blog/installing-openssl-on-ubuntu-16-04-18-04/ Yay!

Finally managed to compile the rest:
```
sudo apt install libsodium-dev # one more cr**p to install

pip install --global-option=build_ext --global-option="-L/usr/local/ssl/lib" --upgrade git+https://github.com/Gallopsled/pwntools.git
```

And of course one more thing was missing:
```
pip install python-dateutil
```

And what I get for all this effort:
```
$ python -c 'from pwn import *'
[!] Pwntools does not support 32-bit Python. Use a 64-bit release.
```

So please, please add a big huge bolded text saying that 32-bits are not supported....