State in README that we backport the 3.12 version of `Protocol`, `runtime_checkable` and various runtime-checkable protocols (#143)

README.md

diff --git a/README.md b/README.md
index d23dbd4..b7e6a7a 100644
--- a/README.md
+++ b/README.md
@@ -92,6 +92,7 @@
   - `TypedDict` (see [PEP 589](https://peps.python.org/pep-0589/))
   - `get_origin` (`typing_extensions` provides this function only in Python 3.7+)
   - `get_args` (`typing_extensions` provides this function only in Python 3.7+)
+  - `SupportsIndex`
 
 - In `typing` since Python 3.7
 
@@ -126,6 +127,17 @@
   - `NamedTuple` (supports multiple inheritance with `Generic` since Python 3.11)
   - `TypeVar` (see PEPs [695](https://peps.python.org/pep-0695/) and [696](https://peps.python.org/pep-0696/))
 
+The following runtime-checkable protocols have always been present in `typing`,
+but the `isinstance()` checks against the `typing_extensions` versions are much
+faster on Python <3.12:
+
+  - `SupportsInt`
+  - `SupportsFloat`
+  - `SupportsComplex`
+  - `SupportsBytes`
+  - `SupportsAbs`
+  - `SupportsRound`
+
 # Other Notes and Limitations
 
 Certain objects were changed after they were added to `typing`, and
@@ -156,6 +168,16 @@
 - `TypeVar` gains two additional parameters, `default=` and `infer_variance=`,
   in the draft PEPs [695](https://peps.python.org/pep-0695/) and [696](https://peps.python.org/pep-0696/), which are being considered for inclusion
   in Python 3.12.
+- `Protocol` was added in Python 3.8, but several bugfixes have been made in
+  subsequent releases, as well as significant performance improvements to
+  runtime-checkable protocols in Python 3.12. `typing_extensions` backports the
+  3.12+ version to Python 3.7+.
+- `SupportsInt`, `SupportsFloat`, `SupportsComplex`, `SupportsBytes`,
+  `SupportsAbs` and `SupportsRound` have always been present in the `typing`
+  module. Meanwhile, `SupportsIndex` was added in Python 3.8. However,
+  `isinstance()` checks against all these protocols were sped up significantly
+  on Python 3.12. `typing_extensions` backports the faster versions to Python
+  3.7+.
 - `Literal` does not flatten or deduplicate parameters on Python <3.9.1, and a
   caching bug was fixed in 3.10.1/3.9.8. The `typing_extensions` version
   flattens and deduplicates parameters on all Python versions, and the caching