Add information about versions in README (#172)

Author: amousset

README.md

@@ -4,7 +4,25 @@ This crate contains the logic to build OpenSSL and is intended to be consumed by
 the `openssl-sys` crate. You likely in theory aren't interacting with this too
 much!
 
-### Windows MSVC Assembly
+## Versioning
+
+There are currently two maintained branches, matching the
+two [maintained OpenSSL versions](https://www.openssl.org/policies/releasestrat.html):
+* `main` which builds OpenSSL 3.0
+* `release/111` which builds OpenSSL 1.1.1
+
+The crates versions follow the `X.Y.Z+B` pattern:
+* The major version `X` is the upstream OpenSSL version:
+  * `111` for 1.1.1
+  * `300` for 3.0 (to be higher than `111`)
+* The minor `Y` and patch `Z` versions are incremented when making changes
+  to the crate, either OpenSSL update or internal changes.
+* `B` contains the full upstream OpenSSL version, like `1.1.1k` or `3.0.7`.
+  Note that this field is actually ignored in comparisons and only there for
+  documentation.
+
+## Windows MSVC Assembly
+
 Building OpenSSL for `windows-msvc` targets, users can choose whether to enable
 assembly language routines, which requires [nasm](https://www.nasm.us/).  
 The build process will automatically detect whether `nasm.exe` is installed in
@@ -14,15 +32,15 @@ the `no-asm` option will NOT be configured).
 You can manipulate this behavior by setting the `OPENSSL_RUST_USE_NASM` environment
 variable:
 * `1`: Force enable the assembly language routines. (panic if `nasm.exe` is not
-availible.)
+available.)
 * `0`: Force disable the assembly language routines even if the `nasm.exe` can be
 found in PATH.
 * not set: Let the build process automatically detect whether `nasm.exe` is
 installed. If found, enable. If not, disable.
 
 However, this environment variable does NOT take effects on non-windows platforms.
 
-# License
+## License
 
 This project is licensed under either of