Link to other resources instead of inlining their information

The other places are more accurate and up-to-date.

- Link to `std-dev-guide` in CONTRIBUTING.md

  Thom and Mara said the guide is in reasonably good shape, and it's tailored more closely to people working on the standard library.

- Link to CONTRIBUTING.md instead of rustc-dev-guide in the main readme

  CONTRIBUTING.md has more information and also links the std-dev-guide.

- Link to forge for the list of tested platforms; the one in the readme
  was hopelessly out of date.

CONTRIBUTING.md

@@ -8,9 +8,9 @@ members](https://rust-lang.zulipchat.com/#narrow/stream/122652-new-members)
 Zulip stream. We have lots of docs below of how to get started on your own, but
 the Zulip stream is the best place to *ask* for help.
 
-Documentation for contributing to Rust is located in the [Guide to Rustc Development](https://rustc-dev-guide.rust-lang.org/),
-commonly known as the [rustc-dev-guide]. Despite the name, this guide documents
-not just how to develop rustc (the Rust compiler), but also how to contribute to the standard library and rustdoc.
+Documentation for contributing to the compiler or tooling is located in the [Guide to Rustc
+Development][rustc-dev-guide], commonly known as the [rustc-dev-guide]. Documentation for the
+standard library in the [Standard library developers Guide][std-dev-guide], commonly known as the [std-dev-guide].
 
 ## About the [rustc-dev-guide]
 
@@ -35,6 +35,7 @@ refer to [this section][contributing-bug-reports] and [open an issue][issue temp
 
 [Contributing to Rust]: https://rustc-dev-guide.rust-lang.org/contributing.html#contributing-to-rust
 [rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org/
+[std-dev-guide]: https://std-dev-guide.rust-lang.org/
 [contributing-bug-reports]: https://rustc-dev-guide.rust-lang.org/contributing.html#bug-reports
 [issue template]: https://github.com/rust-lang/rust/issues/new/choose
 [internals]: https://internals.rust-lang.org

README.md

@@ -6,11 +6,7 @@ standard library, and documentation.
 [Rust]: https://www.rust-lang.org
 
 **Note: this README is for _users_ rather than _contributors_.
-If you wish to _contribute_ to the compiler, you should read the
-[Getting Started][gettingstarted] section of the rustc-dev-guide instead.
-You can ask for help in the [#new members Zulip stream][new-members].**
-
-[new-members]: https://rust-lang.zulipchat.com/#narrow/stream/122652-new-members
+If you wish to _contribute_ to the compiler, you should read [CONTRIBUTING.md](CONTRIBUTING.md) instead.
 
 ## Quick Start
 
@@ -227,41 +223,20 @@ precompiled "snapshot" version of itself (made in an earlier stage of
 development). As such, source builds require an Internet connection to
 fetch snapshots, and an OS that can execute the available snapshot binaries.
 
-Snapshot binaries are currently built and tested on several platforms:
-
-| Platform / Architecture                     | x86 | x86_64 |
-|---------------------------------------------|-----|--------|
-| Windows (7, 8, 10, ...)                     | ✓   | ✓      |
-| Linux (kernel 3.2, glibc 2.17 or later)     | ✓   | ✓      |
-| macOS (10.7 Lion or later)                  | (\*) | ✓      |
-
-(\*): Apple dropped support for running 32-bit binaries starting from macOS 10.15 and iOS 11.
-Due to this decision from Apple, the targets are no longer useful to our users.
-Please read [our blog post][macx32] for more info.
-
-[macx32]: https://blog.rust-lang.org/2020/01/03/reducing-support-for-32-bit-apple-targets.html
+See https://doc.rust-lang.org/nightly/rustc/platform-support.html for a list of supported platforms.
+Only "host tools" platforms have a pre-compiled snapshot binary available; to compile for a platform
+without host tools you must cross-compile.
 
 You may find that other platforms work, but these are our officially
 supported build environments that are most likely to work.
 
 ## Getting Help
 
-The Rust community congregates in a few places:
-
-* [Stack Overflow] - Direct questions about using the language.
-* [users.rust-lang.org] - General discussion and broader questions.
-* [/r/rust] - News and general discussion.
-
-[Stack Overflow]: https://stackoverflow.com/questions/tagged/rust
-[/r/rust]: https://reddit.com/r/rust
-[users.rust-lang.org]: https://users.rust-lang.org/
+See https://www.rust-lang.org/community for a list of chat platforms and forums.
 
 ## Contributing
 
-If you are interested in contributing to the Rust project, please take a look
-at the [Getting Started][gettingstarted] guide in the [rustc-dev-guide].
-
-[rustc-dev-guide]: https://rustc-dev-guide.rust-lang.org
+See [CONTRIBUTING.md](CONTRIBUTING.md).
 
 ## License