Make zigup configurable through an enviroment

README.md

@@ -10,7 +10,7 @@ Otherwise, you can manually find and download/extract the applicable archive fro
 
 # Usage
 
-```
+```sh
 # fetch a compiler and set it as the default
 zigup <version>
 zigup master
@@ -26,6 +26,12 @@ zigup default
 # set the default compiler
 zigup default <version>
 
+# set the default compiler from a path
+zigup default zig/build
+
+# unset the default compiler (for using a global installation)
+zigup undefine
+
 # list the installed compiler versions
 zigup list
 
@@ -41,11 +47,12 @@ zigup run <version> <args>...
 
 # How the compilers are managed
 
-zigup stores each compiler in a global "install directory" in a versioned subdirectory.  On posix systems the "install directory" is `$HOME/zig` and on windows the install directory will be a directory named "zig" in the same directory as the "zigup.exe".
+`zigup` stores each compiler in `$ZIGUP_INSTALL_DIR`, in a versioned subdirectory. The default install directory is `$HOME/.zigup/cache`.
+
+`zigup` makes the zig available by creating a symlink at `$ZIGUP_INSTALL_DIR/<version>` and `$ZIGUP_DIR/default` which points to the current active default compiler.
 
-zigup makes the zig program available by creating an entry in a directory that occurs in the `PATH` environment variable.  On posix systems this entry is a symlink to one of the `zig` executables in the install directory.  On windows this is an executable that forwards invocations to one of the `zig` executables in the install directory.
+Configuration on done during the first use of zigup and the generated environment is installed at `$ZIGUP_DIR/env`.
 
-Both the "install directory" and "path link" are configurable through command-line options `--install-dir` and `--path-link` respectively.
 # Building
 
 Run `zig build` to build, `zig build test` to test and install with:
@@ -56,11 +63,12 @@ cp zig-out/bin/zigup BIN_PATH
 
 # Building Zigup
 
-Zigup is currently built/tested using zig 0.12.0.
+Zigup is currently built/tested using zig 0.13.0+.
 
 # TODO
 
-* set/remove compiler in current environment without overriding the system-wide version.
+- [ ] Download to memory
+- [ ] Use `std.tar` (Unix)
 
 # Dependencies