Only build docs for some MCUs on docs.rs

Sadly we are now starting to hit the limits of docs.rs with the amount
of supported MCUs.  Only build documentation for a subset there and
mention that docs for the rest can be built locally.

Ref: #98

Author: Rahix

Cargo.toml

@@ -19,7 +19,7 @@ include = [
 ]
 
 [package.metadata.docs.rs]
-all-features = true
+features = ["docsrs"]
 
 [features]
 device-selected = []
@@ -52,6 +52,11 @@ attiny88 = ["device-selected"]
 attiny1614 = ["device-selected"]
 rt = ["avr-device-macros"]
 
+# Unfortunately, we can only build documentation for a subset of the supported
+# MCUs on docs.rs.  If you think a very popular chip is missing from the list,
+# feel free to add it here.
+docsrs = ["rt", "atmega328p", "atmega32u4", "atmega2560", "attiny85", "atmega4809"]
+
 [dependencies]
 bare-metal = "0.2.5"
 vcell = "0.1.2"

src/lib.rs

@@ -28,6 +28,10 @@
 #![cfg_attr(feature = "attiny88", doc = "**attiny88**,")]
 //! and a few things which apply to AVR microcontrollers generally.
 //!
+#![cfg_attr(
+    feature = "docsrs",
+    doc = "**Warning**: The doc-build here on docs.rs is only for a subset of supported chips.  Please build documentation locally if your MCU's registers are not documented here.\n\n"
+)]
 //! Which chips the crate is built for depends on the feature flag used.
 //! The following chips are available (using feature flags of the same name):
 //! * `at90usb1286`