README: point to README-hacking

[Akim Demaille]

I installed this, based on https://github.com/akimd/bison/pull/4.


commit d2a0b57846c1f4376808120d77309400817b1e37
Author: Eric S. Raymond <address@hidden>
Date:   Mon Feb 11 21:50:56 2019 -0500

    README: point to README-hacking
    
    * README (Build from git): New.
    * README-hacking: Describe easier submodule update.

diff --git a/README b/README
index 7fa1bfe9..a54231d7 100644
--- a/README
+++ b/README
@@ -1,7 +1,18 @@
 This package contains the GNU Bison parser generator.
 
 * Installation
-** Build
+** Build from git
+Here are basic installation instructions for a repository checkout:
+
+    $ git submodules update --init
+    $ ./bootstrap
+
+then proceed with the usual 'configure && make' steps.
+
+README-hacking contains more information about building and modifying the
+software.
+
+** Build for tarball
 See the file INSTALL for generic compilation and installation instructions.
 
 Bison requires GNU m4 1.4.6 or later.  See:
diff --git a/README-hacking b/README-hacking
index 0a57038d..172566c0 100644
--- a/README-hacking
+++ b/README-hacking
@@ -133,9 +133,15 @@ Enjoy!
 
 ** Updating
 
-The use of submodules make things somewhat different because git does not
-support recursive operations: submodules must be taken care of explicitly by
-the user.
+If you have git at version 1.8.2 or later, the command
+
+        $ git submodule update --recursive --remote
+
+will be useful for updating to the latest version of all submodules.
+
+Under earlier versions, use of submodules make things somewhat different
+because git does not yet support recursive operations: submodules must be
+taken care of explicitly.
 
 *** Updating Bison