Add RELEASING.md (OpenZeppelin#363)

* add releasing.md

* clarify branch role

RELEASING.md

@@ -0,0 +1,32 @@
+# Releasing
+
+Releasing checklist:
+
+(1) Write a changelog.
+
+(2) Make sure to update SPDX license identifiers. For example:
+
+```cairo
+# SPDX-License-Identifier: MIT
+# OpenZeppelin Contracts for Cairo v0.1.0 (account/Account.cairo)
+```
+
+to
+
+```cairo
+# SPDX-License-Identifier: MIT
+# OpenZeppelin Contracts for Cairo v0.2.0 (account/Account.cairo)
+```
+
+(3) Create a release branch and add a tag to it. This branch can be useful if we need to push a hot fix on top of an existing release in the case of a bug.
+
+```sh
+git checkout -b release-0.2.0
+git tag v0.2.0
+```
+
+(4) Push the tag to the main repository, [triggering the CI and release process](https://github.com/OpenZeppelin/cairo-contracts/blob/b27101eb826fae73f49751fa384c2a0ff3377af2/.github/workflows/python-app.yml#L60).
+
+```sh
+git push origin v0.2.0
+```