Improved git build instructions.

README.md

@@ -23,19 +23,21 @@ Source tarball and built executable releases can be found on the
 homepage and on the github release page, https://github.com/stedolan/jq/releases
 
 If you're building directly from the latest git, you'll need flex,
-bison (3.0 or newer), libtool, make, and autoconf installed.  To get
-regexp support you'll also need to install Oniguruma (note that jq's
-tests require regexp support to pass).  To build, run:
-
-    autoreconf -i   # if building from git
-    ./configure
+bison (3.0 or newer), libtool, make, and autoconf installed.
+To get regexp support you'll also need to install Oniguruma or clone it as a
+git submodule as per the instructions below.
+(note that jq's tests require regexp support to pass).  To build, run:
+
+    git submodule update --init # if building from git to get oniguruma
+    autoreconf -fi              # if building from git
+    ./configure --with-oniguruma=builtin
     make -j8
     make check
 
 To build without bison or flex, add `--disable-maintainer-mode` to the
 ./configure invocation:
 
-    ./configure --disable-maintainer-mode
+    ./configure --with-oniguruma=builtin --disable-maintainer-mode
 
 (Developers must not use `--disable-maintainer-mode`, not when making
 changes to the jq parser and/or lexer.)