Docs: Support for git archive installs (#508)

Explains to how install gitlint using github archive URLs.

Docs for #464

Author: jorisroovers

docs/contributing/releases.md

@@ -16,7 +16,22 @@ of a recently published version can still fail for a few minutes after a new ver
 is replicated to all download mirrors.
 
 To install a dev build of gitlint:
-```sh
+```{.sh .copy}
 # Find latest dev build on https://pypi.org/project/gitlint/#history
 pip install gitlint=="0.19.0.dev68"
 ```
+
+### Git archive installs
+You can also install directly from GitHub source Git archive URLs.
+This can even be done for unmerged commits (pending PRs). It will work as long as you have a commit hash.
+
+```{.sh .copy}
+# Set commit hash to install
+export COMMIT_HASH="345414171baea56c5b2b8290f17a2a13a685274c"
+
+# Install using pinned dependencies
+pip install "gitlint-core [trusted-deps] @ https://github.com/jorisroovers/gitlint/archive/$COMMIT_HASH.tar.gz#subdirectory=gitlint-core"
+
+# Install using looser dependencies
+pip install "https://github.com/jorisroovers/gitlint/archive/$COMMIT_HASH.tar.gz#subdirectory=gitlint-core"
+```

docs/installation.md

@@ -10,7 +10,20 @@ To install gitlint with looser dependency requirements, only install gitlint-cor
 ```{.sh .copy}
 pip install gitlint-core
 ```
-??? "Uninstalling gitlint"
+
+??? info "Installing from GitHub source Git archive URLs"
+    ```{.sh .copy}
+    # Set version to install
+    export GITLINT_VERSION="v0.20.0"
+
+    # Install using pinned dependencies
+    pip install "gitlint-core [trusted-deps] @ https://github.com/jorisroovers/gitlint/archive/$GITLINT_VERSION.tar.gz#subdirectory=gitlint-core"
+    
+    # Install using looser dependencies
+    pip install "https://github.com/jorisroovers/gitlint/archive/$GITLINT_VERSION.tar.gz#subdirectory=gitlint-core"
+    ```
+
+??? info "Uninstalling gitlint"
     To fully uninstall gitlint, you need to remove both `gitlint` and `gitlint-core`:
     ```{.sh .copy}
     pip uninstall gitlint gitlint-core