[Flang] Update Maintainers (llvm#117124)

Move to a markdown file and update maintainers.
This brings the project closer to updated guidance
(https://llvm.org/docs/DeveloperPolicy.html#maintainers). A list of
active and inactive maintainers is provided. Maintainers are also
grouped into lead or component maintainers.

Author: kiranchandramohan

flang/Maintainers.md

@@ -0,0 +1,95 @@
+# Flang maintainers
+
+This file is a list of the
+[maintainers](https://llvm.org/docs/DeveloperPolicy.html#maintainers) for
+Flang
+
+## Active maintainers
+
+### Lead maintainers
+The following people are the active maintainers for the project. Please reach
+out to them for code reviews, questions about their area of expertise, or other
+assistance.
+
+#### All areas not covered by others
+Steve Scalpone \
+[email protected] (email), sscalpone (GitHub), sscalpone (Discourse)
+
+####  Backend (Lowering, Fortran dialects, Codegen)
+Jean Perier \
+[email protected] (email), jeanPerier (GitHub), jeanPerier (Discourse)
+
+### Component maintainers
+These maintainers are responsible for particular high-level components within
+Flang that are typically contained to one area of the compiler.
+
+#### Driver
+Andrzej Warzyński \
+[email protected] (email), banach-space (GitHub), banach-space (Discourse)
+
+Tarun Prabhu \
+[email protected] (email), tarunprabhu (GitHub), tarunprabhu (Discourse)
+
+#### Alias Analysis
+Renaud Kauffmann \
+[email protected] (email), Renaud-K (GitHub), Renaud-K (Discourse)
+
+#### PFT, Block, IEEE support
+Val Donaldson \
+[email protected] (email), vdonaldson (GitHub)
+
+#### Lowering, HLFIR, FIR, Codegen
+Jean Perier \
+[email protected] (email), jeanPerier (GitHub), jeanPerier (Discourse)
+
+Slava Zakharin \
+[email protected] (email), vzakhari (GitHub), szakharin (Discourse)
+
+Tom Eccles \
+[email protected] (email), tblah (GitHub), tblah (Discourse)
+
+Valentin Clement \
+[email protected] (email), clementval (GitHub), clementval (Discourse)
+
+#### OpenMP
+##### CPU support
+Tom Eccles \
+[email protected] (email), tblah (GitHub), tblah (Discourse)
+
+##### OpenMP 6.0
+Krzysztof Parzyszek \
+[email protected] (email), kparzysz (GitHub), kparzysz (Discourse)
+
+##### Target Offload
+Sergio Afonso \
+[email protected] (email), skatrak (GitHub), skatrak (Discourse) 
+
+#### OpenACC
+Valentin Clement \
+[email protected] (email), clementval (GitHub), clementval (Discourse)
+
+Razvan Lupusoru \
+[email protected] (email), razvanlupusoru (GitHub), razvan.lupusoru (Discourse)
+
+#### CUDA Fortran
+Valentin Clement \
+[email protected] (email), clementval (GitHub), clementval (Discourse)
+
+#### Debug
+Abid Qadeer \
+[email protected] (email), abidh (GitHub), abidh (Discourse)
+
+## Inactive Maintainers
+### Lead Maintainers
+#### Backend : (Lowering, FIR, Codegen)
+Eric Schweitz \
+[email protected] (email), schweitzpgi (GitHub), schweitz (Discourse)
+
+### Component Maintainers
+#### Semantics
+Tim Keith
+
+#### OpenMP
+Kiran Chandramohan \
+[email protected] (email), kiranchandramohan (GitHub),  kiranchandramohan (Discourse)
+