make spaCy requirement more explicit

[thomas-onesourceregulatory]

Describe the bug
The requirement for spaCy 2.1.8 should be made more explicit (e.g., in new requirements.txt). Currently, this is only in the benchmarking requirements (e.g., requirements-benchmark.txt).

To Reproduce
Steps to reproduce the behavior:

Install pySBD with Spacy 3+ already installed.

Expected behavior

Installation fails due to conflict with spaCy version.

Additional context

The version of spaCy should be listed in requirements.txt, so that this is caught during initial installation (instead of later on when running the benchmark).

At the very least, this should be mentioned in the README file.

Note: Although a fix for spaCy 3+ has been developed (see pull #97), making the requirements explicit is still important.