Skip second README doc test (requires first to have run and execution order isn't guaranteed)

Author: boydgreenfield

README.md

@@ -73,7 +73,7 @@ for p in 0..4u32 {
 
 * After creation, a B-field can optionally be loaded from a directory containing the produced `mmap` and related files with the `load` function. And once created or loaded, a B-field can be directly queried using the `get` function, which will either return `None`, `Indeterminate`, or `Some(BFieldValue)` (which is currently an alias for `Some(u32)` see [limitations](#⚠️-current-limitations-of-the-rust-bfield-implementation) below for more details):
 
-```rust
+```rust no_run
 use bfield::BField;
 
 // Load based on filename of the first array ".0.bfd"

src/bfield.rs

@@ -312,6 +312,7 @@ mod tests {
     }
 }
 
+// Causes cargo test to run doc tests on all `rust` code blocks
 #[doc = include_str!("../README.md")]
 #[cfg(doctest)]
-pub struct ReadmeDoctests;
+struct ReadmeDoctests;