[PATCH] doc: Add a cautionary note

[Mihir Mehta]

I ran into an issue building coreutils with the "bison++" installed,
which is provided by my GNU/Linux distribution. Since this is an issue
which is not detected by the bootstrap or configure scripts, I noted the
incompatibility for users of similar distributions.
---
 README-prereq | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/README-prereq b/README-prereq
index fdf89fecd..9313a2220 100644
--- a/README-prereq
+++ b/README-prereq
@@ -13,8 +13,11 @@ I.e., the tools checked for by the bootstrap script and 
include:
 - Tar       <https://www.gnu.org/software/tar/>
 - Texinfo   <https://www.gnu.org/software/texinfo/>
 
-Note please try to install/build official packages for your system.
-If these programs are not available use the following instructions
+Note: please try to install/build official packages for your system.
+Also, if using Debian or a derived GNU/Linux distribution, please
+install the bison package, since the suggested bison++ package is
+incompatible with building coreutils. If official packages are not
+available for your system, use the following instructions
 to build them and install the results into a directory that you will
 then use when building this package.
 
-- 
2.17.1