added README file with very brief docs

diff --git a/sapi/README b/sapi/README
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+Each SAPI backend has to contain:
+
+config.m4: just like for extensions, this file contains
+autoconf/automake directives that end up in the configure script.  The
+only difference is that the sapi config.m4 files are read in diversion
+(output block) 2 instead of 3.  The sapi config.m4 files should set
+two variables: PHP_SAPI (which sapi backend to choose) and SAPI_TARGET
+(the name of the resulting library or program, previously BINNAME).
+If they are not specified, they will default to "cgi" and "php",
+respectively.
+
+Makefile.inc: has to exist, has to define "INSTALL_IT" to the command
+used to install the final target (or ":" for no operation).  It also
+has to define a plain Makefile rule (without autoconf substitutions)
+to build $(SAPI_TARGET)
+
+Makefile.am: just what you think.  Make sure your target is called
+"libphpsapi_NNN.a", where NNN is the value of PHP_SAPI.