Windows "MAX_PATH" 260 characters limit and Rez.

Hi,

I came across this one while building Boost with variant nesting, basically it is quite easy to hit the Windows "MAX_PATH" 260 characters limit:

``` python
D:\Documents\Development\ThirdParty\boost (master)
λ rez-release

--------------------------------------------------------------------------------
Releasing boost-1.63.0...
--------------------------------------------------------------------------------
Checking state of repository...

--------------------------------------------------------------------------------
Releasing variant 1/1...
--------------------------------------------------------------------------------
Traceback (most recent call last):
File "C:\Program Files\Anaconda2\Lib\runpy.py", line 174, in _run_module_as_main
"__main__", fname, loader, pkg_name)
File "C:\Program Files\Anaconda2\Lib\runpy.py", line 72, in _run_code
exec code in run_globals
File "C:\Program Files\rez\Scripts\rez\rez-release.exe\__main__.py", line 2, in
File "c:\progra~1\rez\lib\site-packages\rez-2.13.0-py2.7.egg\rez\cli\_main.py", line 118, in run
returncode = run_cmd()
File "c:\progra~1\rez\lib\site-packages\rez-2.13.0-py2.7.egg\rez\cli\_main.py", line 110, in run_cmd
return opts.func(opts, opts.parser, arg_groups[1:])
File "c:\progra~1\rez\lib\site-packages\rez-2.13.0-py2.7.egg\rez\cli\release.py", line 138, in command
variants=opts.variants)
File "c:\progra~1\rez\lib\site-packages\rez-2.13.0-py2.7.egg\rezplugins\build_process\local.py", line 76, in release
release_message=release_message)
File "c:\progra~1\rez\lib\site-packages\rez-2.13.0-py2.7.egg\rez\build_process_.py", line 184, in visit_variants
result = func(variant, **kwargs)
File "c:\progra~1\rez\lib\site-packages\rez-2.13.0-py2.7.egg\rezplugins\build_process\local.py", line 220, in _release_variant
install=True)
File "c:\progra~1\rez\lib\site-packages\rez-2.13.0-py2.7.egg\rezplugins\build_process\local.py", line 116, in _build_variant_base
shutil.rmtree(variant_build_path)
File "C:\Program Files\Anaconda2\Lib\shutil.py", line 247, in rmtree
rmtree(fullname, ignore_errors, onerror)
File "C:\Program Files\Anaconda2\Lib\shutil.py", line 247, in rmtree
rmtree(fullname, ignore_errors, onerror)
File "C:\Program Files\Anaconda2\Lib\shutil.py", line 247, in rmtree
rmtree(fullname, ignore_errors, onerror)
File "C:\Program Files\Anaconda2\Lib\shutil.py", line 247, in rmtree
rmtree(fullname, ignore_errors, onerror)
File "C:\Program Files\Anaconda2\Lib\shutil.py", line 247, in rmtree
rmtree(fullname, ignore_errors, onerror)
File "C:\Program Files\Anaconda2\Lib\shutil.py", line 247, in rmtree
rmtree(fullname, ignore_errors, onerror)
File "C:\Program Files\Anaconda2\Lib\shutil.py", line 247, in rmtree
rmtree(fullname, ignore_errors, onerror)
File "C:\Program Files\Anaconda2\Lib\shutil.py", line 252, in rmtree
onerror(os.remove, fullname, sys.exc_info())
File "C:\Program Files\Anaconda2\Lib\shutil.py", line 250, in rmtree
os.remove(fullname)
WindowsError: [Error 3] The system cannot find the path specified: 'D:\\Documents\\Development\\ThirdParty\\boost\\build\\platform-windows\\arch-AMD64\\python-2.7.13\\boost\\libs\\geometry\\doc\\doxy\\doxygen_output\\html_by_doxygen\\classboost_1_1geometry_1_1detail_1_1turns_1_1filter__continue__turns_3_01_turns_00_01true_01_4-members.html'
```

I want to document it as people might face it at some point. my workaround in that particular case was to delete the build directory manually and prepend `\\\\?\\` to any paths I use for the build/release process.

I was debating about patching Rez directly to handle that but doing the above is enough for now.

Start with the Windows traceback from rez-release, especially the cleanup path reaching shutil.rmtree. Locate the relevant user-facing documentation entry point and document the MAX_PATH symptom, manual build-directory cleanup, and the \\?\ workaround described here. Done means a Windows user can find and follow the documented workaround.

Written by the indexing model from the issue text.