Add README file for base-macpro baseline images

Add README file for base-macpro baseline images
Committed: http://code.google.com/p/skia/source/detail?r=2797

[epoger, 2011-12-05 16:17:01]

Note that I get some pixel mismatches against these baselines when I run on my
MacPro.  Maybe some/all of my mismatches are due to system settings?  If so,
what are those system settings so I can add them to the README?

~/src/skia/white/trunk$ out/Debug/gm -r gm/base-macpro
reading from gm/base-macpro
[...]
drawing... lcdtext [640 480]
-----  pixel mismatch for lcdtext_8888 at [154 7] base 0xFFFEFEFE current
0xFFFFFFFF
-----  pixel mismatch for lcdtext_gpu at [155 7] base 0xFFEEFBFF current
0xFFFFFFFF
drawing... hairmodes [640 480]
drawing... gradtext [500 480]
-----  pixel mismatch for gradtext_8888 at [44 74] base 0xFF001D0B current
0xFF000100
[...]
drawing... emptypath [600 280]
-----  pixel mismatch for emptypath_8888 at [92 7] base 0xFFFEFEFE current
0xFFFFFFFF
-----  pixel mismatch for emptypath_gpu at [93 8] base 0xFF89B1DC current
0xFF9B9B9B
[...]

[reed1, 2011-12-05 16:27:27]

lgtm