document "tails" and "continuations" options in readme.txt

readme.txt

@@ -76,7 +76,8 @@ certain flags described below, all of which are optional.
 
  $ make platform={linux,windows,darwin} arch={i386,x86_64,powerpc} \
      process={compile,interpret} mode={debug,debug-fast,fast,small} \
-     bootimage={true,false} heapdump={true,false}
+     bootimage={true,false} heapdump={true,false} tails={true,false} \
+     continuations={true,false}
 
   * platform - the target platform
       default: output of $(uname -s | tr [:upper:] [:lower:]),
@@ -86,7 +87,7 @@ certain flags described below, all of which are optional.
       default: output of $(uname -m), normalized in some cases
       (e.g. i686 -> i386)
 
-  * mode - which set of compilation flags to use, which determine
+  * mode - which set of compilation flags to use to determine
     optimization level, debug symbols, and whether to enable
     assertions
       default: fast
@@ -105,6 +106,18 @@ certain flags described below, all of which are optional.
     heapdump.cpp for details.
       default: false
 
+  * tails - if true, optimize each tail call by replacing the caller's
+    stack frame with the callee's.  This convention ensures proper
+    tail recursion, suitable for languages such as Scheme.  This
+    option is only valid for process=compile builds.
+      default: false
+
+  * continuations - if true, support continuations via the
+    avian.Continuations methods callWithCurrentContinuation and
+    dynamicWind.  See Continuations.java for details.  This option is
+    only valid for process=compile builds.
+      default: false
+
 These flags determine the name of the directory used for the build.
 The name always starts with ${platform}-${arch}, and each non-default
 build option is appended to the name.  For example, a debug build with