Document autoconf dependency in README

The boot script calls autoreconf on line 204. If not present this gives the error message

[...]
Booting .
Can't exec "autoreconf": No such file or directory at boot line 204, <PKGS> line 2.
Running autoreconf failed with exitcode -1 at boot line 204, <PKGS> line 2.

For those following the README to build GHC for the first time this is unfriendly. I had to Google to find that I had to install the autoconf package (on Ubuntu).

The dependency on autoconf should be documented in README.

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System
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Document autoconf dependency in README