Move performance doc to docs/ and rst format.

PERFORMANCE

@@ -1,3 +0,0 @@
-Places for improvement, ordered by difficulty / effectiveness:
-* read_zlib() should have a C equivalent (~ 4% overhead atm)
-* unpack_object() should have a C equivalent

docs/performance.txt

@@ -0,0 +1,7 @@
+Possible areas for improvement
+==============================
+
+Places for improvement, ordered by difficulty / effectiveness:
+
+ * read_zlib() should have a C equivalent (~ 4% overhead atm)
+ * unpack_object() should have a C equivalent