README.md: added section on semantic versioning
README.md: added section on semantic versioning

and added more info on how to generate
the repo for new versions of Unicode and CLDR.

Change-Id: I16c0cf2ed546e57da0490454cfeb8df95289c2e1
Reviewed-on: https://go-review.googlesource.com/63530
Run-TryBot: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

golang/text