Add contributing guidelines for CI/CD and versioning

Expanded CONTRIBUTING.md with detailed guidelines:
- Added purpose and intent to help contributors avoid build issues.
- Documented workflow reuse conventions with examples.
- Defined versioning/tagging rules aligned with SemVer/NuGet.
- Provided CI version sanitization steps and usage examples.
- Included troubleshooting tips for common CI issues.
- Emphasized maintenance alignment with GitHub Actions/NuGet.

Author: maythamfahmi

CONTRIBUTING.md

@@ -0,0 +1,100 @@
+# Contributing Guidelines
+
+## Purpose
+
+This document explains project-level development and CI/CD conventions
+contributors must follow to avoid common build and packaging failures.
+
+## Workflow Reuse Guidelines
+
+### Calling a Reusable Workflow
+
+-   When invoking a reusable workflow using `uses:` at **job level**,
+    **do NOT specify `runs-on`** in the caller.
+-   The **reusable workflow must**:
+    -   Declare `on: workflow_call`
+    -   Define `runs-on` for its internal jobs
+-   Inputs are passed via `with:`, and secrets via `secrets:`.
+
+### Example: Caller (no `runs-on`)
+
+``` yaml
+jobs:
+  call-build:
+    uses: ./.github/workflows/cd-build-test-pack.yml
+    with:
+      version: ${{ github.ref_name }}
+      configuration: Release
+```
+
+### Example: Reusable Workflow
+
+``` yaml
+on:
+  workflow_call:
+    inputs:
+      version:
+        required: true
+        type: string
+      configuration:
+        required: true
+        type: string
+        default: Release
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      # …
+```
+
+## Versioning and Git Tag Guidelines
+
+-   Tags used for packaging and publishing---whether passed to
+    `/p:Version=` or `dotnet pack`---**must be valid SemVer/NuGet
+    version strings**.
+-   **Do NOT include a leading `v`** when passing the version into
+    `dotnet` commands.
+
+### Valid Examples
+
+-   `3.2.6`
+-   `3.2.6-preview`
+-   `3.2.6-preview20251130229`
+-   `3.2.6-alpha.1+build.123`
+
+### Invalid Examples
+
+-   `v3.2.6-preview20251130229`
+-   `3.2.6 preview`
+
+### Recommended Tagging Practice
+
+-   Tag with leading `v` locally if desired, but CI must sanitize it.
+
+## CI Behavior and Version Sanitization
+
+### Sanitization Example
+
+``` yaml
+- name: Sanitize version
+  id: sanitize
+  run: |
+    v="${{ inputs.version }}"
+    v="${v#v}"
+    v="${v#V}"
+    echo "sanitized_version=$v" >> "$GITHUB_OUTPUT"
+```
+
+Use via: `${{ steps.sanitize.outputs.sanitized_version }}`
+
+## Troubleshooting
+
+-   If CI reports "not a valid version string," check for leading `v` or
+    invalid characters.
+
+## Maintenance
+
+-   Keep conventions aligned with GitHub Actions and NuGet rules.