Merge #534: Add build error and README docs for proper use of std and no-std features

2f84d32 Add README build docs for std and no-std (Steve Myers)
d1c71e3 Add build error if both std and no-std features disabled (Steve Myers)

Pull request description:

  Building this crate requires the std and/or no-std features be enabled. This PR documents this build constraint in the README and gives an error is anyone tries to build without enabling one or both of these features.

  See discussion in #533.

ACKs for top commit:
  Kixunil:
    ACK 2f84d32
  apoelstra:
    ACK 2f84d32

Tree-SHA512: b25606739caa4f3a586f55cf774aa824a836f17fd19412087243d6d0b62f46900031c39d8f8bce1d5d42857b2dd0c9a5d6d6e7a559abd3aad250251f77dd9ef5

Author: apoelstra

README.md

@@ -33,6 +33,11 @@ coins in a given Bitcoin transaction
 More information can be found in [the documentation](https://docs.rs/miniscript)
 or in [the `examples/` directory](https://github.com/apoelstra/rust-miniscript/tree/master/examples)
 
+## Building
+
+The cargo feature `std` is enabled by default. At least one of the features `std` or `no-std` or both must be enabled.
+
+Enabling the `no-std` feature does not disable `std`. To disable the `std` feature you must disable default features. The `no-std` feature only enables additional features required for this crate to be usable without `std`. Both can be enabled without conflict.
 
 ## Minimum Supported Rust Version (MSRV)
 This library should always compile with any combination of features (minus

src/lib.rs

@@ -90,6 +90,9 @@ compile_error!(
     "rust-miniscript currently only supports architectures with pointers wider than 16 bits"
 );
 
+#[cfg(not(any(feature = "std", feature = "no-std")))]
+compile_error!("at least one of the `std` or `no-std` features must be enabled");
+
 pub use bitcoin;
 
 #[cfg(not(feature = "std"))]