update readme

README.txt

@@ -37,8 +37,30 @@ both `setup.sh` and `shrc`.
 Sourcing flow
 -------------
 
-When using `sh`, rc file for interactive use is found by `ENV` and it is usually
-set to `$HOME/.shrc`.
+1. $HOME/.profile
+
+Sourced if:
+currnet shell is login shell and
+    (current shell is sh ||
+     (current shell is bash && $HOME/.bash_profile does not exist) ||
+     (current shell is zsh && $HOME/.zprofile does not exist))
+
+This file set ENV to $HOME/.shrc , which is loaded when sh is run interactively.
+Add lines like `test -n "$BASH_VERSION" && . "$HOME/.bashrc"` .
+
+
+3. $HOME/.shrc
+
+Sourced through ENV when shell is sh, and $HOME/.{ba,z}shrc if bash or zsh
+respectively. Source $HOME/.dotfiles/shrc .
+
+
+3. $HOME/.{ba,z}shrc
+
+Sourced when current shell is bash or zsh respectively and current shell is not
+login shell. When current shell is login shell, these files are sourced
+explicitly by $HOME/.bash_profile or $HOME/.zprofile .
+
 
 
 $HOME/.{ba,z}shrc     <= bash, zsh
@@ -54,5 +76,3 @@ $HOME/.{ba,z}shrc     <= bash, zsh
    |--~/.dotfiles/xprofile
    |
    `--~/.dotfiles/xprograms
-
-