WIP: feat(doc): Add a doc for beginners

doc/for-beginners.md

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+# Enable full `rust-analyzer` support of `cg_gcc` for your editor
+
+## Steps to Follow
+1. (Already done in by project.) Add the following lines to `Cargo.toml`
+```toml
+[package.metadata.rust-analyzer]
+rustc_private = true
+```
+2. Install rust-src with rustup: `rustup component add rust-src`.
+   NOTE: Make sure you've switched to the corresponding toolchain as is used by cg_gcc
+3. Set the lsp option `rust-analyzer.rustc.source = "discover"` (you can also set it to an explicit path), depending on your editor.
+    1. Neovim(nvim-lspconfig):
+    ```
+    require('lspconfig').rust_analyzer.setup{
+      settings = {
+        ['rust-analyzer'] = {
+          -- add this section
+          rustc = {
+            source = "discover",
+          },
+        },
+      },
+    }
+    ```
+   2. Emacs: customize `Lsp Rust Analyzer Rustc Source` to the Path `discover`
+
+## FAQ
+
+### Why is this needed?
+
+Without these steps, `rust-analyzer` will NOT import `rustc` related packages to its scope and all rustc related clauses will fail to be analyzed in lsp, and the analyzer will notify you of the missing `rustc_*` packages.
+
+### Why is this NOT needed for `rustc_codegen_llvm` in your `rust-src`?
+In the `Cargo.toml` of the `rustc_codegen_llvm`, you may notice the following lines:
+```toml
+rustc_ast = { path = "../rustc_ast" }
+rustc_attr = { path = "../rustc_attr" }
+rustc_codegen_ssa = { path = "../rustc_codegen_ssa" }
+rustc_data_structures = { path = "../rustc_data_structures" }
+rustc_errors = { path = "../rustc_errors" }
+rustc_fluent_macro = { path = "../rustc_fluent_macro" }
+rustc_fs_util = { path = "../rustc_fs_util" }
+rustc_hir = { path = "../rustc_hir" }
+rustc_index = { path = "../rustc_index" }
+rustc_llvm = { path = "../rustc_llvm" }
+rustc_macros = { path = "../rustc_macros" }
+rustc_metadata = { path = "../rustc_metadata" }
+rustc_middle = { path = "../rustc_middle" }
+rustc_query_system = { path = "../rustc_query_system" }
+rustc_session = { path = "../rustc_session" }
+rustc_span = { path = "../rustc_span" }
+rustc_symbol_mangling = { path = "../rustc_symbol_mangling" }
+```
+They set the paths to the rustc-related packages, solving the problem of `rustc_private` packages.
+
+In theory, it is possible that you use the same workaround as in `rustc_codegen_llvm`.
+
+While the `rustc_codegen_cranelift` uses the same approach as `rustc_codegen_gcc`.
+
+```
+[package.metadata.rust-analyzer]
+rustc_private = true
+```
+
+# Compiling External Project with `rustc_codegen_gcc`
+
+1.  Run the program with (in the root dir of `rustc_codegen_gcc` )(where `--manifest-path` is the path to the `Cargo.toml` of the project to run `cargo` against):
+
+```shell
+    ./y.sh cargo --manifest-path="..."
+```
+
+or you may as well add this alias in your `.bash_aliases`:
+
+
+```shell
+    alias cargcc="/path/to/rustc_codegen_gcc/y.sh cargo"
+```
+
+2.  If std or libcore is needed, then the sysroot should be built first(see `build_system/src/test.rs` for hint.)