Add comment, README note on orjson Python 3.10 wheels

Author: duckontheweb

.github/workflows/continuous-integration.yml

@@ -39,6 +39,10 @@ jobs:
           python-version: ${{ matrix.python-version }}
 
       - uses: actions-rs/toolchain@v1
+        # No wheels exist for orjson on Python 3.10 on MacOS or Windows, and the Linux
+        #  wheels are sometimes not uploaded until after the package is initially
+        # published. This sets up the Rust nightly toolchain so we can build the orjson
+        # wheel.
         if: ${{ startsWith(matrix.python-version, '3.10')}}
         with:
           toolchain: nightly

README.md

@@ -37,6 +37,10 @@ optional `orjson` requirements:
 > pip install pystac[orjson]
 ```
 
+> *`orjson` wheels are only available for Linux in Python 3.10. If you are using the
+> `orjson` extra with Python 3.10 you will need to have the Rust nightly toolchain
+> installed as your default toolchain in order to build the package wheel.*
+
 From source repository:
 
 ```bash