docs: document version of crustls dependency

This also pins a specific release in the Travis test so future
API-breaking changins in crustls won't break curl builds.

Add RUSTLS documentation to release tarball.

Enable running tests for rustls, minus FTP tests (require
connect_blocking, which rustls doesn't implement) and 313 (requires CRL
handling).

Closes #6763

docs/RUSTLS.md

@@ -0,0 +1,26 @@
+# Rustls
+
+[Rustls is a TLS backend written in Rust.](https://docs.rs/rustls/). Curl can
+be built to use it as an alternative to OpenSSL or other TLS backends. We use
+the [crustls C bindings](https://github.com/abetterinternet/crustls/). This
+version of curl depends on version v0.4.0 of crustls.
+
+# Building with rustls
+
+First, [install Rust](https://rustup.rs/).
+
+Next, check out, build, and install the appropriate version of crustls:
+
+    % cargo install cbindgen
+    % git clone https://github.com/abetterinternet/crustls/ -b v0.4.0
+    % cd crustls
+    % make
+    % make DESTDIR=${HOME}/crustls-built/ install
+
+Now configure and build curl with rustls:
+
+    % git clone https://github.com/curl/curl
+    % cd curl
+    % ./buildconf
+    % ./configure --without-ssl --with-rustls=${HOME}/crustls-built
+    % make