docs/release.md: Add release documentation

docs/release.md

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+# Release process
+
+This project follows [semantic versioning](https://semver.org/). The following
+documentation will refer to `X.Y.Z` as _major_, _minor_ and _patch_ version.
+
+## Development between releases
+
+- Every substantial pull request should add an entry to the `[unreleased]`
+  section of the corresponding crate `CHANGELOG.md` file. See
+  [#1698](https://github.com/libp2p/rust-libp2p/pull/1698/files) as an example.
+
+  In case there is no `[unreleased]` section yet, create one with an increased
+  major, minor or patch version depending on your change. In addition update the
+  version in the crate's `Cargo.toml` as well as the corresponding entry of the
+  crate in the root level `Cargo.toml`.
+
+
+## Releasing one or more crates
+
+1. Remove the `[unreleased]` tag for each crate to be released in the respective
+   `CHANGELOG.md` and create a pull request against the rust-libp2p `master`
+   branch.
+
+2. Once merged, create and push a tag for each updated crate.
+
+    ```bash
+    cd $CRATE-PATH
+    tag="$(sed -En 's/^name = \"(.*)\"$/\1/p' Cargo.toml)-$(sed -En 's/^version = \"(.*)\"$/\1/p' Cargo.toml)"
+    # Use `-s` for a GPG signed tag or `-a` for an annotated tag.
+    git tag -s "${tag}" -m "${tag}"
+    git push origin "${tag}"
+    ```
+
+3. Create and push a tag for the top level `libp2p` crate, if it is being
+   released.
+
+    ```bash
+    cd $REPOSITORY-ROOT
+    # Note the additional `v` here.
+    tag="v$(sed -En 's/^version = \"(.*)\"$/\1/p' Cargo.toml)"
+    git tag -s "${tag}" -m "${tag}"
+    git push origin "${tag}"
+    ```
+
+4. Publish each tagged crate to crates.io. `cargo` assists in getting the order
+   of the releases correct.
+
+    ```
+    cd <CRATE-SUBDIRECTORY>
+    cargo publish
+    ```