fixing readme and configure script links to double-conv

Summary:
Fixing following the issues raised by nsuke on github
following his diff (https://github.com/facebook/folly/pull/112)

Test Plan: visual inspection, tested build on arch linux

Reviewed By: davejwatson@fb.com

Subscribers: folly-diffs@

FB internal diff: D1807378

Tasks: 5909189

Signature: t1:1807378:1422403093:ca024f396a27ecd9b304c57643f0107764a38ca1

README

@@ -77,13 +77,16 @@ of by the bootstrap script.)
 Other Linux distributions
 -------------------------
 
-- double-conversion (http://code.google.com/p/double-conversion/)
+- double-conversion (https://github.com/floitsch/double-conversion/)
 
   Download and build double-conversion.
   You may need to tell configure where to find it.
 
-  ./configure LDFLAGS=-L$DOUBLE_CONVERISON_HOME/ CPPFLAGS=-I$DOUBLE_CONVERISON_HOME/src/
+  [double-conversion/] ln -s src double-conversion
  
+  [folly/] ./configure LDFLAGS=-L$DOUBLE_CONVERISON_HOME/ CPPFLAGS=-I$DOUBLE_CONVERISON_HOME/
+ 
+  [folly/] LD_LIBRARY_PATH=$DOUBLE_CONVERISON_HOME/ make
 
 - additional platform specific dependencies: