[pseudo] Add readme

[hokein]

Thanks!

clang/lib/Tooling/Syntax/Pseudo/README.md

9

syntactic => semantic I think.

clang/lib/Tooling/Syntax/Pseudo/README.md

+# clang pseudoparser
+
+This directory implements an approximate heuristic parser for C++, based on the
+clang lexer, the C++ grammar, and the GLR parsing algorithm.
+
+It parses a file in isolation, without reading its included headers.
+The result is a strict syntactic tree whose structure follows the C++ grammar.
+There is no semantic analysis, apart from guesses to disambiguate the parse.
+Disambiguation can optionally be guided by an AST or a symbol index.
+
+For now, the best reference on intended scope is the [design proposal],
+with further discussion on the [RFC].
+
+## Dependencies between pseudoparser and clang
+
+Dependencies are limited because they don't make sense, but also to avoid
+placing a burden on clang mantainers.
+
+The pseudoparser reuses the clang lexer (clangLex and clangBasic libraries) but
+not the higher-level libraries (Parse, Sema, AST, Frontend...).
+
+When the pseudoparser should be used together with an AST (e.g. to guide
+disambiguation), this is a separate "bridge" library that depends on both.
+
+Clang does not depend on the pseudoparser at all. If this seems useful in future
+it should be discussed by RFC.
+
+## Parity between pseudoparser and clang
+
+The pseudoparser aims to understand real-world code, and particularly the
+languages and extensions supported by Clang.
+
+However we don't try to keep these in lockstep: there's no expectation that
+Clang parser changes are accompanied by pseudoparser changes or vice versa.
+
+[design proposal]: https://docs.google.com/document/d/1eGkTOsFja63wsv8v0vd5JdoTonj-NlN3ujGF0T7xDbM/edit
+[RFC]: https://discourse.llvm.org/t/rfc-a-c-pseudo-parser-for-tooling/59217/49