Support pinned tag to protect struct fields from overriding

[igkon]

PR for issue #242.
Adding support of pinned tag to protect struct fields from overriding during merge with mergo.WithOverride option.
Also adding documentation for the feature to README.md.

Test is in issue242_test.go

[darccio]

@igkon Not sure about this one. I understand it's to maintain some values "protected" against overwrite, but the name "pinned" doesn't feel related to that operation.

Also, a struct tag doesn't seem like the best option. You may want in the same project not to overwrite that field in one place, and maybe in other you need to do it.

I'm closing as, additionally, mergo's code is frozen. Only bugs are pulled into main.