Add --skip-validation on xmlto on Make docs

Trying to build the docs on my MacOS 10.9, and xmlto keeps failing because of validation issues.
After a bit of online research I saw other people having the same
issue. We should skip-validation for now until all xml docs are valid so people wont have the same issue.

This is the validation error when trying to `make doc`
```
make[2]: `GIT-VERSION-FILE' is up to date.
    XMLTO git-add.1
xmlto: /Users/arthurnn/dev/git/Documentation/git-add.xml does not validate (status 3)
xmlto: Fix document syntax or use --skip-validation option
I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
/Users/arthurnn/dev/git/Documentation/git-add.xml:3: warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
                   "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
                                                                            ^
I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
validity error : Could not load the external subset "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
Document /Users/arthurnn/dev/git/Documentation/git-add.xml does not validate
make[1]: *** [git-add.1] Error 13
make: *** [install-doc] Error 2
```