Merge pull request #6812 from smcv/64-bit-required

README: Document x86-64 requirement

README.md

@@ -7,9 +7,15 @@ Hardware and Software Requirements
 ----------------------------------
 Steam for Linux requires the following:
 
-- 1 GHz Pentium 4 or AMD Opteron with 512 megabytes of RAM and 5 gigabytes of hard drive space, or better
+- 1 GHz Pentium 4 or AMD Opteron with:
+    - x86-64 (AMD64) instruction set
+    - CMPXCHG16B instruction support (`cx16` in `/proc/cpuinfo` flags)
+    - SSE3 instruction support (`pni` in `/proc/cpuinfo` flags)
+- 512 megabytes of RAM and 5 gigabytes of hard drive space, or better
 - Internet connection (Cable/DSL speeds recommended)
 - Latest Ubuntu LTS, fully updated
+- 64-bit (x86-64, AMD64) Linux kernel
+- 64-bit (x86-64, AMD64) *and* 32-bit (i386, IA32) graphics drivers and glibc
 - Latest graphics driver
 - NVidia driver support - For recent cards (e.g. series 8), you will need to install 310.x. For older cards, driver 304.x supports the NVidia 6 and 7 GPU series. To access these drivers, first update your cache and then install the specific [driver](https://support.steampowered.com/kb_article.php?ref=8509-RFXM-1964) you need from the list in Additional Drivers.
 - AMD driver support - For recent cards (e.g. series 5 and above), we recommend installing the 12.11 driver. For older cards, Catalyst 13.1 Legacy supports the HD 2400 Pro card and is the latest for the 2 and 4 GPU series.