Add recommendation note

[nemoinho]

#550 Add note that it is not recommended to use --no-metadata because it may breaks

synchronisation between git and svn.

[ben]

Hmm. Ordinarily I'd say this is good, but since this section is about doing a one-way conversion to Git, we may not actually want the metadata in the commit messages. Can we reword the warning to be something more like this?

You can also tell git svn not to include the metadata that Subversion normally imports, by passing --no-metadata to the clone or init command (though if you want to keep the metadata, feel free to omit this parameter).

[nemoinho]

Yes, this is also ok, but let me add one more word to your suggestion:

You can also tell git svn not to include the metadata that Subversion normally imports, by passing --no-metadata to the clone or init command (though if you want to keep the synchronisation-metadata, feel free to omit this parameter).

So I changed the pull-request like that.

[ben]

Nice. Thanks!