README tweaks

- Mention cbindgen
- Mention acknowledgments' README
- Un-mention submodules (the one we had was removed)

Author: jrose-signal

README.md

@@ -49,12 +49,6 @@ On a Debian-like system, you can get these extra dependencies through `apt`:
 $ apt-get install clang libclang-dev cmake make protobuf-compiler git
 ```
 
-Additionally, some of the tests in this repository rely on submodules being checked out:
-
-```shell
-$ git submodule update --init
-```
-
 The build currently uses a specific version of the Rust nightly compiler, which
 will be downloaded automatically by cargo. To build and test the basic protocol
 libraries:
@@ -66,6 +60,9 @@ $ cargo test
 ...
 ```
 
+When making a PR that adjusts dependencies, you'll need to regenerate our acknowledgments files.
+See [``acknowledgments/README.md``](acknowledgments/).
+
 ## Java/Android
 
 To build for Android you must install several additional packages including a JDK,
@@ -92,7 +89,11 @@ $ make
 ```
 
 When exposing new APIs to Java, you will need to run `rust/bridge/jni/bin/gen_java_decl.py` in
-addition to rebuilding.
+addition to rebuilding. This requires installing the `cbindgen` Rust tool:
+
+```shell
+$ cargo +stable install cbindgen
+```
 
 ### Maven Central
 

swift/README.md

@@ -42,8 +42,11 @@ When exposing new APIs to Swift, you will need to add the `--generate-ffi` flag
 2. Use `swift build` and `swift test` as usual from within the `swift/` directory.
 
 When exposing new APIs to Swift, you will need to add the `--generate-ffi` flag to your
-`build_ffi.sh` invocation.
+`build_ffi.sh` invocation. This requires installing the `cbindgen` Rust tool:
 
+```shell
+$ cargo +stable install cbindgen
+```
 
 ## Use as a Swift Package