README: add instructions for missing INSTALL file

The file INSTALL has been dropped in 2009 with [1] and it has been
replaced by the INSTALL file generated by automake.
The file INSTALL is only present in official releases of OpenOCD
and not in the code cloned from git.

Add a note in README to report this inconsistency.

Change-Id: I881b4282550a80bd5a6c61baff05543714a2e9c5
Reported-by: Mats <[email protected]>
Signed-off-by: Antonio Borneo <[email protected]>
Fixes: https://sourceforge.net/p/openocd/tickets/451/
Link: [1] commit ef733b4 ("Switch automake handling to use
	--gnu mode, not --foreign. Remove INSTALL file; automake
	generates a copy of the latest version.")
Reviewed-on: https://review.openocd.org/c/openocd/+/9121
Tested-by: jenkins

Author: borneoa

README

@@ -191,6 +191,11 @@ and compiling the OpenOCD source code. That file is provided by
 default for all GNU autotools packages. If you are not familiar with
 the GNU autotools, then you should read those instructions first.
 
+Note: if the INSTALL file is not present, it means you are using the
+source code from a development branch, not from an OpenOCD release.
+In this case, follow the instructions 'Compiling OpenOCD' below and
+the file will be created by the first command './bootstrap'.
+
 The remainder of this document tries to provide some instructions for
 those looking for a quick-install.