docs/EXPERIMENTAL: explain what it means and what's experimental now

docs/EXPERIMENTAL.md

@@ -0,0 +1,22 @@
+# Experimental
+
+Some features and functionality in curl and libcurl are considered
+**EXPERIMENTAL**.
+
+Experimental support in curl means:
+
+1. Experimental features are provided to allow users to try them out and
+   provide feedback on functionality and API etc before they ship and get
+   "carved in stone".
+2. You must enable the feature when invoking configure as otherwise curl will
+   not be built with the feature present.
+3. We strongly advice against using this feature in production.
+4. **We reserve the right to change behavior** of the feature without sticking
+   to our API/ABI rules as we do for regular features, as long as it is marked
+   experimental.
+5. Experimental features are clearly marked so in documentation. Beware.
+
+## Experimental features right now
+
+ - HTTP/3 support and options
+ - alt-svc support and options

docs/Makefile.am

@@ -53,6 +53,7 @@ EXTRA_DIST =                                    \
  CODE_STYLE.md                                  \
  CONTRIBUTE.md                                  \
  DEPRECATE.md                                   \
+ EXPERIMENTAL.md                                \
  FAQ                                            \
  FEATURES                                       \
  GOVERNANCE.md                                  \