Add notes about running examples.

src/bare-metal/alloc.md

@@ -17,5 +17,6 @@ To use `alloc` you must implement a
   allocator defined in your binary. Usually this is done in the top-level binary crate.
 * `extern crate panic_halt as _` is necessary to ensure that the `panic_halt` crate is linked in so
   we get its panic handler.
+* This example will build but not run, as it doesn't have an entry point.
 
 </details>

src/bare-metal/useful-crates/aarch64-paging.md

@@ -28,6 +28,7 @@ idmap.activate();
 * For now it only supports EL1, but support for other exception levels should be straightforward to
   add.
 * This is used in Android for the [Protected VM Firmware][2].
+* There's no easy way to run this example, as it needs to run on real hardware or under QEMU.
 
 </details>
 

src/bare-metal/useful-crates/buddy_system_allocator.md

@@ -12,6 +12,8 @@ we might want to allocate MMIO space for PCI BARs:
 <details>
 
 * PCI BARs always have alignment equal to their size.
+* Run the example with `cargo run` under `src/bare-metal/useful-crates/allocator-example/`. (It won't
+  run in the Playground because of the crate dependency.)
 
 </details>
 

src/bare-metal/useful-crates/spin.md

@@ -25,6 +25,7 @@ fn main() {
   from `std::sync`;  and `Lazy` for lazy initialisation.
 * The [`once_cell`][2] crate also has some useful types for late initialisation with a slightly
   different approach to `spin::once::Once`.
+* The Rust Playground includes `spin`, so this example will run fine inline.
 
 </details>
 

src/bare-metal/useful-crates/tinyvec.md

@@ -21,6 +21,7 @@ fn main() {
 <details>
 
 * `tinyvec` requires that the element type implement `Default` for initialisation.
+* The Rust Playground includes `tinyvec`, so this example will run fine inline.
 
 </details>
 

src/bare-metal/useful-crates/zerocopy.md

@@ -17,6 +17,8 @@ working with structures shared with hardware e.g. by DMA, or sent over some exte
 * Attempting to derive `FromBytes` for these types would fail, because `RequestType` doesn't use all
   possible u32 values as discriminants, so not all byte patterns are valid.
 * `zerocopy::byteorder` has types for byte-order aware numeric primitives.
+* Run the example with `cargo run` under `src/bare-metal/useful-crates/zerocopy-example/`. (It won't
+  run in the Playground because of the crate dependency.)
 
 </details>