Added INSTALL files for Darwin and Linux.

INSTALL.darwin

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+To install Oclgrind, simply copy the bin, lib and include directories
+to (for example) /usr/local/, ensuring that file modification times
+are preserved. The easiest way to do this is with the following
+command:
+
+    sudo cp -rp {bin,lib,include} /usr/local
+
+Alternatively, Oclgrind can be used from a non-system directory. To do
+so, add $OCLGRIND_ROOT/bin to your PATH environment variable, and
+$OCLGRIND_ROOT/lib to your DYLD_LIBRARY_PATH environment variable
+(where $OCLGRIND_ROOT is the directory containing this file). If
+copying Oclgrind to a new location, ensure that the -p flag is passed
+to cp, to ensure that file modification times are preserved.
+
+Information about using Oclgrind can be found on the GitHub wiki page:
+
+    http://github.com/jrprice/Oclgrind/wiki

INSTALL.linux

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+To install Oclgrind, simply copy the bin, lib and include directories
+to (for example) /usr/local/, ensuring that file modification times
+are preserved. The easiest way to do this is with the following
+command:
+
+    sudo cp -rp {bin,lib,include} /usr/local
+
+Alternatively, Oclgrind can be used from a non-system directory. To do
+so, add $OCLGRIND_ROOT/bin to your PATH environment variable, and
+$OCLGRIND_ROOT/lib to your LD_LIBRARY_PATH environment variable (where
+$OCLGRIND_ROOT is the directory containing this file). If copying
+Oclgrind to a new location, ensure that the -p flag is passed to cp,
+to ensure that file modification times are preserved.
+
+To use Oclgrind with the OpenCL ICD loader (optional), copy
+oclgrind.icd to /etc/OpenCL/vendors/.
+
+Information about using Oclgrind can be found on the GitHub wiki page:
+
+    http://github.com/jrprice/Oclgrind/wiki