Update README.md

Author: devernay

README.md

@@ -102,16 +102,18 @@ Note on Xcode versions
 
 Here are the latest versions of Xcode that are known to /run/ on each OS X version (the links work if you [sign in to Apple Developer](https://developer.apple.com/downloads/) first):
 
-- [Xcode 2.5](http://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_2.5_developer_tools/xcode25_8m2558_developerdvd.dmg) on Mac OS X 10.4 (Tiger)
-- [Xcode 3.1.4](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_3.1.4_developer_tools/xcode314_2809_developerdvd.dmg) on Mac OS X 10.5 (Leopard)
-- [Xcode 3.2.6](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_3.2.6_and_ios_sdk_4.3__final/xcode_3.2.6_and_ios_sdk_4.3.dmg) on Mac OS X 10.6 (Snow Leopard) - [Xcode 4.0.2](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4.0.2_and_ios_sdk_4.3/xcode_4.0.2_and_ios_sdk_4.3.dmg), [Xcode 4.1](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4.1_for_snow_leopard_21110/xcode_4.1_for_snow_leopard.dmg) and [Xcode 4.2](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4.2_with_ios_5_sdk/xcode_4.2_and_ios_5_sdk_for_snow_leopard.dmg) also run on Snow Leopard, but are only available to pay members 
+- [Xcode 2.5](http://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_2.5_developer_tools/xcode25_8m2558_developerdvd.dmg) on Mac OS X 10.4 (Tiger) (*)
+- [Xcode 3.1.4](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_3.1.4_developer_tools/xcode314_2809_developerdvd.dmg) on Mac OS X 10.5 (Leopard) (*)
+- [Xcode 3.2.6](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_3.2.6_and_ios_sdk_4.3__final/xcode_3.2.6_and_ios_sdk_4.3.dmg) on Mac OS X 10.6 (Snow Leopard) - [Xcode 4.0.2](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4.0.2_and_ios_sdk_4.3/xcode_4.0.2_and_ios_sdk_4.3.dmg), [Xcode 4.1](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4.1_for_snow_leopard_21110/xcode_4.1_for_snow_leopard.dmg) and [Xcode 4.2](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4.2_with_ios_5_sdk/xcode_4.2_and_ios_5_sdk_for_snow_leopard.dmg) also run on Snow Leopard, but are only available to pay members (*)
 - [Xcode 4.6.3](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4.6.3/xcode4630916281a.dmg) on OS X 10.7 (Lion)
 - [Xcode 5.1.1](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_5.1.1/xcode_5.1.1.dmg) on OS X 10.8 (Mountain Lion)
 - [Xcode 6.2](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/Xcode_6.2/Xcode_6.2.dmg) on OS X 10.9 (Mavericks)
 - [Xcode 7.2.1](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/Xcode_7.2.1/Xcode_7.2.1.dmg) on OS X 10.10 (Yosemite)
 - [Xcode 7.3.1](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/Xcode_7.3.1/Xcode_7.3.1.dmg) on OS X 10.11 (El Capitan), please see note on linking below. [Xcode 8.2.1](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/Xcode_8.2.1/Xcode_8.2.1.xip) also runs on OS X 10.11, but can only compile for macOS 10.12.
 - [Xcode 8.3.3](https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/Xcode_8.3.3/Xcode8.3.3.xip) on macOS 10.12 (Sierra), please see note on linking below.
 
+(*) These Xcode versions were released before 26.03.2012 and may cause an "An unknown installation error" during installation, which is due to an expired certificate. Installing these may require disabling network time sync and setting the date to 01.01.2012 before installing. Network time sync may be re-enabled after install. See [this stackexchange question](https://apple.stackexchange.com/questions/45841/xcode-4-2-snow-leopard-doesnt-install) for more details.
+
 More information about the compilers included in each version of Xcode can be found on the [MacPorts Wiki](https://trac.macports.org/wiki/XcodeVersionInfo) and on [Wikipedia](https://en.wikipedia.org/wiki/Xcode).
 
 ### Linking for x86_64 on Xcode 4.4 and later