readme: Add Troubleshooting section

Describe common solutions/answers to problems with v compilation.

README.md

@@ -139,6 +139,36 @@ sudo dnf install glfw glfw-devel freetype-devel
 
 glfw dependency will be removed soon.
 
+
+## Troubleshooting:
+
+You can see how v invokes the C backend compiler with `v -show_c_cmd source.v` .
+
+You can produce a .c file, *without* compiling it further with `v -o file.c file.v` . 
+That is useful, if you want to integrate v as a transpiler into the build system (probably using a Makefile) of an existing large C code base, or if you just want to read the produced C code.
+
+You can prevent v from deleting the intermediate .c file (which is useful if you want to use a debugger like gdb or msvc) by: `v -debug file.v` .
+
+You can pass `-g`, which has the effect of -debug, and in addition will make the debugger information to have v line numbers, instead of C ones (NB: this will make the intermediate .c file harder to read).
+
+
+You can also set the VFLAGS environment variable to pass one or more flags to v, so that you do not have to type them manually everytime.
+Windows (cmd): `set VFLAGS=-debug -show_c_cmd`
+Windows (PowerShell): `$env:VFLAGS="-debug -show_c_cmd"`
+Unix (bash): export VFLAGS="-debug -show_c_cmd"
+
+Windows:
+If you get this error while running the v repl, and you are using msvc:
+`'gcc' is not recognized as an internal or external command, operable program or batch file.`
+
+... please try:
+```shell
+set VFLAGS=-os msvc
+v.exe runrepl
+
+```
+
+
 ## Contributing
 
 Code structure: