DOC: split the README into README + INSTALL

The README was barely usable after all the additions having accumulated
over the years. This patch introduces a new INSTALL file explaining how
to build and install haproxy with various levels of details. The README
is now mostly an index to the list of useful documentations.
diff --git a/scripts/create-release b/scripts/create-release
index afd6e56..68781b2 100755
--- a/scripts/create-release
+++ b/scripts/create-release
@@ -188,8 +188,8 @@
 sed -ne '0,/^%changelog/d;p' < examples/haproxy.spec >>examples/haproxy.spec-
 mv examples/haproxy.spec- examples/haproxy.spec
 
-# updating branch and date in README and all modified doc files except the outdated architecture.txt
-for file in README doc/intro.txt doc/configuration.txt doc/management.txt $(git diff --name-only v${OLD}.. -- doc); do
+# updating branch and date in all modified doc files except the outdated architecture.txt
+for file in doc/intro.txt doc/configuration.txt doc/management.txt $(git diff --name-only v${OLD}.. -- doc); do
 	if [ ! -e "$file" ]; then continue; fi
 	if [ "$file" = doc/architecture.txt ]; then continue; fi
 	echo "Updating $file ..."
@@ -208,7 +208,7 @@
 
 if [ -n "$INTERACTIVE" ]; then
 	vi CHANGELOG VERSION VERDATE examples/haproxy*.spec \
-	   src/haproxy.c README doc/configuration.txt \
+	   src/haproxy.c doc/configuration.txt \
 	   $(git diff --name-only v${OLD}.. -- doc)
 fi