feat(#3957): Mention xcop in README

[volodya-lombrozo]

Since we need xcop to be installed before building the project, we need to mention it in the docs.
Moreover, recent bugs show that we need at least 0.8.0 version of xcop to build the project.

Closes: #3957

[github-actions]

🚀 Performance Analysis

Test	Base Score	PR Score	Change	% Change	Unit	Mode
benchmarks.XmirBench.xmirToEO	155.370	167.200	11.830	7.61%	ms/op	Average Time
benchmarks.XmirBench.xmirToPhi	170.034	166.835	-3.199	-1.88%	ms/op	Average Time
benchmarks.XmirBench.xmirToSaltyPhi	129.141	156.270	27.129	21.01%	ms/op	Average Time

⚠️ Performance loss: benchmarks.XmirBench.xmirToEO is slower by 11.830 ms/op (7.61%)
✅ Performance gain: benchmarks.XmirBench.xmirToPhi is faster by 3.199 ms/op (1.88%)
⚠️ Performance loss: benchmarks.XmirBench.xmirToSaltyPhi is slower by 27.129 ms/op (21.01%)

[maxonfjvipon]

@volodya-lombrozo FYI, it's not mandatory that xcop is installed on your local machine. If it's not installed - the all xcop steps are just skipped during the build. But if it's installed - it must be >= 0.8.0

[volodya-lombrozo]

@volodya-lombrozo FYI, it's not mandatory that xcop is installed on your local machine. If it's not installed - the all xcop steps are just skipped during the build. But if it's installed - it must be >= 0.8.0

@maxonfjvipon I see, thanks. Let me fix the description then.

[volodya-lombrozo]

@maxonfjvipon Is it better now?

[maxonfjvipon]

@volodya-lombrozo definitely, thanks!