[gen] add README.md to document diy7 filter.

Author: Shale Xiong

gen/README.md

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+A online tutorial can be found in this [link](https://diy.inria.fr) that covers `diy7`, `diycross7` and `diyone7`.
+We document some extra information here.
+
+# Tool `diy7` internal filter
+Tool `diy7` enforce some filter by default, that is, function `choice_default` in source code file [gen/alt.ml](alt.ml).
+Given an unfinished cycle $[r_1, r_2, r_3, ..., r_n]$, the function defines next allowed primitive relaxation $r_{n+1}$. 
+In the case of compound relaxation, it checks against the first primitive relaxation, e.g. $[r_{n+1}, r_{n+2}, ..., r_k]$.
+If the filter `choice_default` passes, returning `true`, the new unfinished/finished cycle will be, $[r_1, r_2, r_3, ..., r_n, r_{n+1}]$ or $[r_1, r_2, r_3, ..., r_n, r_{n+1}, r_{n+2}, ..., r_k]$.
+Before expaining the filter, let introduce some terminology on relaxations, or specifically, edges, where concrete examples are from `-arch AArch64`:
+
+- internal communication edge, namely `Coi`, `Fri` and `Rfi`
+- external (communication) edge, namely `Coe`, `Fre`, `Rfe` and `Hat`
+- insert edge, that is, fence without memory events, e.g., `DMB.SY` and `DMB.LD`
+- dependency edge, e.g. `DpAddrdW` and `DpCtrlsR`
+- program order edge that connects two memory events without any extra restrict, e.g. `PosRW` and `PodWR`
+- read-modify-write (RMW) edge, e.g. `LxSx` and `Amo.Cas`.
+
+Given two primitive relaxations, `lhs` and `rhs`, the following cases pass the filter `choice_default lhs rhs`:
+
+| `lhs` | `rhs` |
+|-------|-------|
+| internal communication edge or insert edge | dependency edge or program order edge between different loations such as `PodWR` |
+| dependency edge or program order edge between different loations such as `PodWR` | internal communication edge or insert edge |
+| dependency edge between different locations such as `DpAddrdW` | program order edge between different loations such as `PodWR` |
+| program order edge between different loations such as `PodWR` | dependency edge between different locations such as `DpAddrdW` |
+| internal read-from edge `Rfi` | program order edge between the same location such as `PosWR` |
+| program order edge between the same location such as `PosWR` | internal read-from edge `Rfi` |
+| RMW edge | any edge |
+| any edge | RMW edge |
+| external edge | any edge |
+| any edge | external edge |