doc: Add document for vmtop

FAQ: explanation of some terms
TODO: something to do in future
HOWTO: ways to devalop
Signed-off-by: NJiajun Chen <1250062498@qq.com>

doc/FAQ

+1. Explanations of terms in project.
+ 1) field
+    Member of task that is being to show in screen
+    ===========================================
+      DID        VMname     PID  %CPU    S    P
+      3            cjj2   64716   0.0    S   23
+      |_       qemu-kvm   64716   0.0    S   23
+      |_       qemu-kvm   64734   0.0    S   13
+    ===========================================
+    "DID", "VMname" are field names, and "3" is the DID field of task,
+    "cjj2" is VMname field of task.
+ 2) delay time
+    time unit 1s, time interval between two display, and is also time interval
+    for obtaining data.
+
+2. What does items vmtop display means?
+    DID: virtual machine id in libvirt
+    VM/task-name: name of virtual machine or threads
+    PID: pid of qemu or threads
+    %CPU: CPU usage of task
+    EXTxxx: kvm exit times
+    P: physical cpu that task runs
+    S: task state, include: R, S, D, T, Z, X
+    %ST: cpu time vcpu wait pcpu while it is running other vcpu
+    %GUE: cpu time in guest
+    %HYP: cpu time in hypervisor
+
+3. What archs are supported?
+   Currently, only aarch64 is supported.
+
+4. Where to send bug report?
+   You may send your bug report to virt@openeuler.org or open an issue in
+   https://gitee.com/openeuler/vmtop.

doc/HOWTO

+1.How to add a display item for vmtop?
+Of course, the display item data get should have been realized. Then, you can add related display item to vmtop display through the following steps:
+ 1) Add a enum item in fields_type(field.h).
+    Take 'S' display item as example:
+    Add "FD_STATE" enum in fields_type
+    --------------------------------
+    enum fields_type {
+        FD_DID,
+        ...
+        FD_STATE,
+        FD_END
+    };
+    --------------------------------
+ 2) Add display setting in fields(field.c):
+    --------------------------------
+    FID fields[] = {
+        /* name  .      flag    . align */
+        {"DID",   FIELDS_DISPLAY, 5  },
+        ...
+        {"S",     FIELDS_DISPLAY, 5  }
+    };
+    --------------------------------
+ 3) Add relate print case in print_domain_field(vmtop.c):
+    --------------------------------
+    static void print_domain_field(struct domain *dom, int field)
+    {
+        int i = field;
+        switch (i) {
+        case FD_VMNAME: {
+    |       printw("%*.*s", fields[i].align, fields[i].align - 2, dom->vmname);
+    |       break;
+        }
+        ...
+        case FD_STATE: {
+    |       printw("%*c", fields[i].align, dom->state);
+    |       break;
+        }
+        ...
+    }
+    --------------------------------
+ 4) Make and run.

doc/TODO

+1. Disk io status
+   Show disk io status for each disk of each virtual machine.
+
+2. Set io status
+   Show net io status for each net card of each virtual machine.
+
+3. Sort key
+   Add short cut key to choose which field to sort by.
+
+4. Introducing multithreads
+   Now, vmtop use delay time to set time interval between two data acquisitions.