Documentation to mention CGO_ENABLED=0 is not possible

[dinamic]

It seems the library is of no help when targetting static binaries. Let's have the documentation updated to indicate this.

/app # CGO_ENABLED=1 time go build .
real	0m 0.13s
user	0m 0.17s
sys	0m 0.10s

/app # CGO_ENABLED=0 time go build .
github.com/kolesa-team/go-webp/encoder: build constraints exclude all Go files in /go/pkg/mod/github.com/kolesa-team/go-webp@v1.0.4/encoder
github.com/kolesa-team/go-webp/decoder: build constraints exclude all Go files in /go/pkg/mod/github.com/kolesa-team/go-webp@v1.0.4/decoder
Command exited with non-zero status 1
real	0m 0.14s
user	0m 0.19s
sys	0m 0.12s
/app #