docs: add Used By section to README

[shenxianpeng]

Summary

Adds a "Used By" section to the README, showcasing notable organizations and projects that use the xianpengshen/clang-tools Docker image.

Notable users included

Organization	Description
LLNL	Lawrence Livermore National Laboratory (US DOE) — via ExaEpi project
CLEARSY	Systems engineering company specializing in formal methods for safety-critical systems
Mumble	Popular open-source VoIP application
AutoBleem	PlayStation Classic modding project
ValeevGroup	Computational chemistry research group (Virginia Tech)
ada-url	WHATWG-compliant URL parser adopted by Node.js

Rationale

• Consistent with cpp-linter-action and cpp-linter-hooks repos which already have "Used By" sections
• Provides social proof and credibility for potential users evaluating the Docker image
• Links to full GitHub code search for xianpengshen/clang-tools so readers can discover all dependents

Preview

LLNL   CLEARSY   Mumble   AutoBleem   ValeevGroup   ada-url   and many more

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud