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/.gitignore b/.gitignore
index 32a8a84..8a38c4f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,6 +7,7 @@
 !/LICENSE
 !/Makefile
 !/README
+!/INSTALL
 !/CONTRIBUTING
 !/MAINTAINERS
 !/ROADMAP